US2424791A

US2424791A - Electron microscope apparatus

Info

Publication number: US2424791A
Application number: US519774A
Authority: US
Inventors: Charles H Bachman; Ramo Simon
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-12-01
Filing date: 1944-01-26
Publication date: 1947-07-29
Anticipated expiration: 1964-07-29
Also published as: GB669381A; US2424789A; FR58024E; GB576003A; BE576003A; BE480179A; US2424788A; BE474126A; FR938893A; BE483238A; GB647703A; US2424790A; US2400331A; FR964837A

Description

y 1947- c. H. BA-CHMAN EI'AL 2,424,791

ELECTRON MICROSCOPE APPARATUS Original Filed Dec. 1, 1942 6 Sheets-Sheet l -4/ I a 75 4 a 4 4 EE 4 4 i a ll] lll/ Inventors: 'L Charl e s H. Bachman, Slmon Ram o, 2 2 by Jr/w z, 1W

Their Attorney July 29, 1947. c. H. BACHMAN HAL ELECTRON MICROSCOPE APPARATUS Original Filed Dec. 1, 1942 6 Sheets-Sheet 2 Inventors Chagjes Hgachman lmon amo.

Their Attorn e July 29, 1947. c. H. BACHMAN ETAL ELECTRON MICROSCOPE APPARATUS Original Filed Dec. 1, 1942 6 Sheets-Sheet 5 Inventors: Chafies H. Bachman,

Simon Ramo Th zAttorney July 29, 1947. c. H. BACHMAN ETAL 2,424,791

ELECTRON MICROSCOPE APPARATUS Original Filed Dec. 1, 1942 e Sheets-Sheet 4 Inventors: Charles H. Bachman. Simon Ramo,

Their Attorney y 1947- c. H. BACHMAN EI'AL 2,424,791

ELECTRON MICROSCOPE APPARATUS Original Filed Dec. 1, 1942 6 Sheets-Sheet 5 Inventors: Charles H. Bachman,

Simon Ramo, b ,Va/m 66144 44 Their Attorney.

y 1947- c. H. BACHMAN ETAL 2,424,791

ELECTRON MICROS COIE APPARATUS Original Filed Dec. 1, 1942 6 Sheets-Sheet 6 Fig. 24.

Inventors:

Charles HBachmzm,

slmon R mo b MAL y Their/Attorney Patented July 29,. 1947 ELECTRON MICROSCOPE APPARATUS Charles H. Baelunan. Seotia, and Simon Ramo,

Niskayuna, N. Y., alllgnora to General E lectric Company, a corporation of New York Original application December 1, 1942, Serial No. 467,530. Divided and this application January 28, 1944, Serial No. 519,774

The present invention which is a division of our copending application 8. N. 467,530, filed December 1, 1942, relates to electron microscopes, and more particularly to new and improved structure for electron microscopes of the electrostatic type.

In electron microscopes of the prior art, and particularly those which have been constructed in accordance with the prior art arrangements and teachings, the structures thereof, particularly the evacuated chamber within which the elements are placed, have not been readily adaptable or susceptible to widespread application due to the fact that the evacuated chambers have been of relatively large size because of the type of the electron lens systems employed, thereby involving relatively large time intervals between examinations of different specimens.

The prior art arrangements have also been subject to the disadvantage of requiring the services of skilled operators to effect frequent realinement of the electrodes or focusing apparatus. Where, in the prior art, the magnetic type of electron microscope has been employed, in order to obtain accuracy of the image produced and to retain a desired degree of precision in the focusing effect, it ha been necessary to use a large number of regulating or controlling elements for controlling the voltage or current supplied to the electrical elements of the microscope. Due to the above factors the size of the prior art microscopes has been inordinately large, thereby restricting the degree of portability and thereby limiting the field of application.

In accordance with the teachings of our invention described hereinafter, we provide a new and improved electron microscope which obviates the disadvantages of the prior art arrangements and which affords simplicity of construction and operation, and adaptability to a wide field of application. The electron microscope which we provide is adapted and arranged to permit frequent opening of the evacuated chambar for the insertion and extraction of specimens to be investigated and which entails only the lapse of a relatively short interval of time between examinations due to the fact that the chamber is relatively small. In addition, the specimen insertion chamber and sealing structure thereof are arranged to permit ease of insertion and extraction of the specimen. Furthermore, the essential elements of the electron microscope, such as the cathode construction and the electron lens system, are readily demountable for inspection 19 Claims. (Cl. 250-495) assembly is provided so that the alinement of the electrodes thereof may be initially made at the factory, thereby obviating the necessity for frequent and time consuming realinement during use after manufacture.

It is an object of our invention to provide a new and improved electron microscope.

' It is still another object of our invention to provide new and improved structure for an electron microscope wherein the essential elements are readily demountable and available for inspection and replacement.

It is a further object of our invention to provide a new and improved electron microscope wherein specimens may be inserted and extracted without involving the lapse of undesirable periods of time between examinations of different specimens.

It is a still further object of our invention to go provide a new and improved specimen carrier for an electron microscope.

It is still another object of our invention to provide a new and improved means for adjusting the position of a specimen in an electron microg5 scope.

appended claims.

For a better understanding of our invention, reference may be had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the Fig. 1 diagrammatically illustrates an embodiment of our invention wherein the evacuated chamber containing the len systern is horizontally disposed and wherein the actuating means for moving the specimen are 10- 86 cated within the vicinity of the viewing aperture.

Fig. 2 is a cross-sectional view of the cabinet supporting the microscope and shows the disposition of associated or auxiliary apparatus. Fig. 3 is a cross-sectional view of the electron microscope 40 showing the evacuated chamber, electron lens 60 perspective view of a manipulator which may be employed in our electron microscope, and Fig. 9

represents the cartridge type specimen carrier. Fig. 10 represents the structure for holding and positioning an electron beam restricting means and replacement. Lastly, a unitary electron lens such as a positional apertured disk. Fig. 11 is a view of the specimen insertion member; 1' 8. 12 is a detailed view of the locket which is supported at one end of the holder shown in Fig. 11; Fig. 13 is a view of the sectionalized horizontal conductor which connects corresponding electrodes of the various horizontally disposed electrostatic lens system. Fig. 14 is an exploded view of the manipulator; Figs. 15 and 16 'are front and side views, respectively, of the manipulator showing the horizontal motion produceable thereby; Figs. 17 and 18 are front and side views, respectively, showing the vertical transverse motion produceable thereby, and Figs. 19 and 20 are front and side views, respectively, of the manipulator showin the longitudinal or focusing action produceable by the manipulator. Fig. 21 is a detailed view of the externally accessible actuating means or wheels which control the specimen manipuator. This figure also shows bellows-type sealing structure and the member for supporting a viewing screen or fluorescent screen. Fig. 22 is a detailed view of the insertable optical or glass lens which may be used in conjunction with our electron microscope. Fig. 23 illustrates the manner in which a camera may be used to photograph the-image'produced upon the fluorescent screen of the electron microscope, and Fig. 24 is a simplified circuit diagram showing the controllable source of voltage for the electron microscope and the cathode filament supply.

Prior to a detailed description of the embodiment of our invention illustrated, it is believed that it may be helpful to present generally certain fundamental aspects of the electron microscope disclosed herein. Generally speaking, the electron microscope which we provide is one employing an electrostatic type lens. The microscope comprises essentially an electron gun which produces an electron beam to illuminate or irradiate a specimen to be investigated and an imaging system which magnifies the image produced by the impingement on the specimen into a larger image on a viewing screen such as a fluorescent screen, or upon the surface of a P graphic plate.

In an electron microscope of the electrostatic type which we provide, the lens focal length is a function of its physical size and configuration since an increase or decrease in the electron velocity is always accompanied by a change in the electric field focusing action of just such magnitude or strength as to yield precisely the same electron paths. By the use of such a system it is then not necessary to employ a closely regulated source of supply voltage for the microscope.

Of course, as is well known, the desirability of employing an electron microscope in place of a light microscope for many applications is the greater resolving power incident to the use of electron irradiation as contrasted with the smaller resolving power of light microscopes. Although not limited to a particular resolving power or magnification, an electron microscope built in accordance with our invention operates satisfactorily having a 200 Angstrom units resolution and a useful magnification of about 10,000x. As pointed out hereinafter, this magnification may be obtained either exclusively by the design of the electron lens system itself, or by the combination of a suitable electron electrostatic lens and an associated light microscope or lens which is placed in the vicinity of the fluorescent screen at one end of the electron microscope.

In the electron microscope described herein- 4 after. certain features are incorporated by virtue of the configuration and position of the elements to obtain the desired operation of the electron gun. the irradiation of the specimen, and the desired focusing action of the electrostatic lens. As

concerns the electron gun, this element is constructed to produce sufllcient current density at the specimen without involving an appreciable heating of the specimen by the electron beam. On the other hand, the image is illuminated to a sumcient degree of brightness for visual observation with a minimum of current striking the specimen. It will be apparent that there is no advantage to irradiate a large part of a specimen if only a small portion on the axis is to be imaged. Furthermore, there is no reason to direct electrons at the specimen if their incident angle is such that in the absence of deflection they will consequently be rejected by the objective lens aperture stop. The contrast obtained at the viewing screen or fluorescent screen is the result of distribution of the electrons arriving there. We have found that for very thin specimens the majority of the electrons which pass through the specimen do so with their slope substantially unchanged; consequently, the limiting angle of the electron bundle leaving the objective lens can be established principally by the electron gun apparatus rather than by the apertures of the obiective lens system. This type of limiting is generally easier to accomplish mechanically by the necessary aperture in the electron gun, and this necessary aperture may be more favorably located than the objective lens stop so as to obviate the need of punching exceedingly small diameter holes in the lens electrodes. Of course, the lens aberrations as well as the particular thickness and material of the specimen that is under examination, play an important part. in determining whether the gun angle can serve as an effective stop for the system, or whether that limit is determined by the first imaging or objective lens. It has been found that highly successful results are obtained with a small gun angle, and the gun angle produces a more readily discernible effect upon the resolving power than does the lens aperture.

The electron microscope described hereinafter is constructed to afi'ord proper relationship between the cathode filament point location, the filament shield aperture diameter, anode-filament spacing and the spacing and size of the other apertures. The spacing and size of the othe apertures of the system depend upon a number of factors such as the location of the imaging lens system and the specimen, the filament life, the accelerating voltage, the electron beam angle at the specimen and the extent of magnetic shielding of the field due to the filament current.

As concerns the electrostatic electron lenses which magnify the electron image produced by the impingement of the electron beam on the specimen, the electrostatic lens system of the electron microscope which we provide possesses the following general characteristics. The focal length of the system is positive and relatively short without involving the use of excessively small parts. The in-focus position of the specimen is external to the high field region of the lens and the specimen is in a region substantial- 1y entirely free from electric fields. The electrodes of the lens system are finely machined or ground and highly polished to minimize the insulation problem and field emission at the operating voltage. The lenses are of symmetrical configuration with a single negative potential to ground, namely the cathode potential, serving to energize the lens. The axial voltage drop in the central region of the lens is maintained at an optimum value, thereby minimizing strong magnetic fields and chromaticaberration effects.

Fig. 1 of the drawings represents a perspective view of a cabinet I, which is of a portable nature being mounted on castors 2, and which houses associated auxiliary apparatus for an electron microscope, and on the front of which the electron microscope is horizontally positioned within a cylindrical cover 3. There is provided a desk or table part 4 integral with the cabinet I to facilitate use of the microscope. An end cover 5 encloses the end of cylindrical cover 3 and is provided with a viewing aperture through which an eye-piece 6 extends. This eye-piece is shown in detail in Fig. 22. There is also positioned within the vicinity of the eye-piece or viewing aperture a plurality of externally accessible actuating means, such as knurledwheels I, 3, and 9 which serve to operate a manipulator (described hereing structure which includes alternate layers of inafter) to position a specimen or object to be I ing means for the electron gun, such as a centrifugal blower i5,'is also positioned in this compartment of the cabinet. In the lower compartment of the cabinet, we employ an evacuation means such as a pair of serially connected pumps I6 and I], both of which are supported by vibration absorption means such as springs I3 and [9. Pump I6 is preferably a high vacuum twostage oil diffusion pump which operates in series witha mechanical pump, the latter of which furnishes the rough vacuum. Control means for the electrical apparatus and for the pumping apparatus may be positioned within the front of the cabinet i. For example, the rheostatic regulator i3 of the voltage supply for the electron gun of the microscope may be provided with a handle or knob 20 which extends through the wall of the cabinet to make it readily accessible to the operator. In like manner, referring to Fig. 1, the filament supply for the cathode of the electron gun may be controlled by a knob or dial 2| also positioned on thefront of the cabinet. Likewise, a main power switch 22 and a handle 23 connected to the vacuum valve l4 through gearing (not shown) may be made easily accessible to the operator by mounting as indicated.

In Fig. 3 there is illustrated a cross-sectional view of the evacuated chamber, electron gun, the electrostatic lens system, and the specimen carrier manipulator and actuating means thereof. The evacuated chamber within which the microscope elements described above are positioned is defined in part by a metallic cylinder, such as a brass cylinder 24, the interior surface of which may be adapted to closely engage an insertable unitary electron lens assembly to be described more in detail hereinafter and which is shown in Fig. '7. As a means for shielding cylinder 24. we provide a laminated metallic shieldple, we may employ cylindrical sheaths of copper 26 and 26 and alternate layers of

high permeability iron

21 and 29. The sheath which is adjacent cylinder 24 may be either iron or copper and may be turned over at its ends in order to secure the other layers in the position illustrated.

A unitary electrostatic electron lens assembly is defined externally by means of a metallic cylinder 29, preferably constructed of brass, the inner surface of which is finely ground and highly polished to closely engage cylinder 24 and to support therein the elements of the electrostatic lens system. The inner surface of cylindner 24 and the outer surface of cylinder 29 need not be as precisely machined as the inner surface of cylinder 29. Within the cylinder 29 we provide an electrostatic lens system which includes a plurality of generally similar lenses 30, 3| and 32. This len assembly may be arranged to have a total magnification of about 1500. Lenses 30 and 3| each comprise a pair of longitudinally spaced wafer-

like electrodes

33 and 34 maintained at a common or anode potential and which are provided with apertures 35 and 36, respectively. We provide an intermediate electrode 31 also of wafer or disk-like configuration in spaced relation between

outer electrodes

33 and 34 and which is provided with an aperture 38 of substantially larger diameter than the apertures 35 and 36.

Electrodes

33 and 34 are constructed to have their peripheries finely ground and polished to closely engage the inner surface of cylinder 29, thereby establishing satisfactory mechanical and electrical connection to the system. Intermediate electrode 31 is electrically insulated from cylinder 29 and is maintained at cathode potential by means of a horizontal conductor connected thereto and shown in Figs. 7 and 13. Electrode 31 is maintained in the position illustrated by means of a split insulating spacer or washer 39 which is supported by and in engagement with the inner surface of cylinder 29. The insulating washer or spacer 39 may be split, constituting two segmental parts to facilitate assembly thereof with the intermediate electrode 31.

It will be noted that the peripheries of apertures 35, 36, and 38 are convex, thereby reducing to a minimum the tendency to establish undesired voltage gradients within the vicnity of the apertures or along the electron lens system.

The various elements of the unitar electron lens assembly are susceptible of precise alinement and locking, thereby permitting an initial alinement of the electrodes at the time of manufacture and obviating the necessity of realinement in the field. For example, the electrodes of the lenses 3032 may be maintained and locked in the desired illustrated position by means of a plurality of principal longitudinal and annular spacers 40-42 inclusive. In addition, the intermediate electrode and the outer electrodes may be maintained in the desired spaced relation by means of smaller

annular spacers

43 and 44 preferably of metal.

In accordance with our invention, there is provided also a unitary structure comprising the aforementioned electron lens system and a specimen carrier 45, shown in Fig. 9, and a specimen manipulator 46, shown in Fig. 8. The

elements

45 and 46 will be explained hereinafter in detail. The specimen carrier 45 is supported by manipulator 46 which in turn is supported from (5 one end of the lens assembly,

The objective electron lens I! is essentially of the same electrical characteristics as electron lenses 30 and Si and comprises a pair of spaced outer electrodes 41 and 48 and an intermediate electrode 49. Outer electrode 48 is provided with an aperture 48' and a radial recess adapted to receive a specimen holder shown in Fig. 11, and is also provided with an enlarged aperture adapted to receive one part oi the cartridge type specimen carrier shown in Fig.9.

As a. part of the unitary construction comprising the electron lens assembly and the carrier and manipulator, we also may add an electron beam defining means, such as an apertured disk 50 and supporting structure therefor shown in Fig. 10.

This supporting structure includes means for centering and positioning the disk 50 which may be constructed of molybdenum and which serves to produce a well defined beam for the desired illumination of the specimen or object which is held by carrier 45. The apertured disk 50 is preferably of such dimension to subtend at the specimen a solid angle in the neighborhood of 10- radians.

A radial specimen insertion chamber 54 is provide in alinement with slot 5| to permit ready insertion and extraction of a specimen in carrier 45. The insertion chamber may be defined by a radial tubular member 52 preferably constructed of nonmagnetic material and supporting at its upper end a valve 55 for sealing the insertion chamber 54 and also for sealing the main evacuated chamber of the microscope. Valve 55 comprises a removable cover 56 which is engaged by a cross rod which when pressure is exerted thereagainst maintains the cover firmly against the upper surface of a cylinder 58, constituting the side wall defining means of the valve. If desired, a suitable gasket 55, such as a rubber gasket, ma be employed to seal the juncture of the cover 56 and cylinder 56.

Means are provided for supplying to the evacuated chamber of the microscope air at atmospheric pressure prior to the opening of cover 55. This means may comprise a passage 60 fed by a conduit 6i through which preconditioned or heated air may flow to prevent the condensation of appreciable moisture on the internal parts of the microscope when it is desired to extract or insert a specimen. is a means for selectively controlling the flow of the heated air, we provide a valve 62 seated in passage 60 and actuated by a member 63. A bellows-type sealing member 64 is sealed around the adjacent area of cylinder 58 and arm 63. The actuating mechanism for cover 56 and valve 62 is shown in Fig, 6.

At one end of the cylinder 24 there is located a sealing and supporting structure for a filamentary cathode comprising one of the electrodes of an electron gun, which comprises a filamentary pointed cathode 65 of the hairpin type. There is also provided magnetic shielding structure comprising apertured wafer-like metallic members or disks 66 and 61 which are firmly positioned against the inner surface of the cylinder 24. Electrostatic shielding means, such as a transverse metallic planar member 61' is positioned between members 66 and 61 intermediate the electron beam apertures of th latter members and the position of a longitudinal conductor to be described presently, thereby defining a substantially field-free region through which the electron beam passes. Locking means are provided for maintaining all the elements of the system in a fixed longitudinal and angular position with respect to the unitary assembly described hereinafter. This locking means may comprise a longitudinal key 68 which engages an abutment of cylinder 29 or extends into a recess provided thereby, and aLso extends axially through Openings at the peripheries of disks 66 and 61. Key 66 may also extend through

annular sp era

66 and 10 which maintain disks 6! and 66 and the unitary assembly in the desired spaced relation.

Disks 66, and 61 are provided with apertures TI and 12 through which a horizontal conductor shown in Fig. 13 extends to impress on the intermediate electrodes of electron lenses 30, ll and 32 a potential corresponding to the cathode potential. That is, the cathode structure to be described hereinafter and the intermediate electrodes are all maintained at the same potential. :Ii' desired, there may be employed an acceleratmg anode cup structure 13 supported by a transverse disk II' and extending an appreciable distance longitudinally around the filamentary cathode 65 so that a suitable accelerating electric field is provided for the electrons emitted by the cathode.

Exhaust apparatus for the evacuated chamber or the electron microscope may be connected to a tubular conduit 15 in communication with the chamber and which is also connected to the pumps I6 and I1 shown in Fig. 2.

In order to support the cathode structure and to seal one end of the evacuated chamber delined by cylinder 24, we provide a supporting ring 16 which is seated upon a shoulder provided by cylinder 24 and which also engages an annular spacer H. A sealing structure is employed and comprises a compressible washer, such as a rubber washer 18, a. metallic compression ring 18, a

' threaded retaining ring 80 adapted to engage threads provided at the end of cylinder 24, and a plurality of circumferentially displaced means, such as screws 6|, for exerting longitudinal or axial pressure against the retaining ring 19, thereby compressing Washer 16.

A vitreous part 82 is sealed to the supporting ring 18 and serves to support and seal the cathode structure. The vitreous part 62 may be of the re-entrant type through which a concentric transmission line comprising an inner conductor 83 and a tubular outer conductor 84 extends. The details of the cathode and supporting structure will be discussed with respect to Fig. 4.

At the other end of the evacuated chamber, particularly at the left-hand end of cylinder 24, there is provided an end wall for sealing that end of the chamber and for supporting a viewing screen, such as a fluorescent screen 85 coated on the inner surface with a suitable fluorescent material and upon which the magnified image is produced by the impingement of the electron beam. This end wall comprises an apertured disk 66 which seats the viewing screen or fluorescent screen 85. A suitable seal for the screen 85 may be obtained by means of a rubber washer 6'! which is compressed by means of a flanged annular ring 86, the latter being attached to disk 66. The outer periphery or disk 86 is also sealed in a similar manner by means of a. compressible rubber washer 89, a metallic compression ring -90, and a retaining ring ill, and pressure exerting means such as screws 92 which exert an actual pressure on compression ring 90, thereby compressing the washer 69 and sealing the juncture.

Means for actuating or controlling the specimen manipulator u are also positioned at end or the evacuation chamber within the vi cinity or the viewing screen The manipulator. maybeofthetypedesignedtoproducethr'ee degrees or motion mutually at right each other, and may comprise three separate acspring III which is supported at its right-hand extremity by means oi a threaded collar III tuating means such as longitudinal extending

rods

93, 34, and 06. only two oi which are shown in Fig. 3 and which are controlled by of a plurality of screw-thread devices ll. 61,

",only two 01' which are shownin Fig. 3. The screw-thread device I! is shown in cross-seccylindrical insulator I" placed between retaintional detail and is also representative of the other two devices. For example, screw-thread device 91 comprises a head It which engages the associated longitudinal actuating rod 44 and is controlled in its longitudinal position by means of a screw Hill which is provided, with an actuate ing pin IM and which has one end thereof extending through disk II to elect an externally accessible connection adapted to receive a flexible cable shown in Fig. 21. Each screw-thread device may be supported by a cylindrical member I42 formed integral with'or attached to'disk -36 which is sealed to its associated head 46 by a deformable member such as a bellows I63 sometimes deflned as a Sylphon bellows.

Reference is now made to Fig. 4 in which the cathode supporting structure is shown in greater detail. Means are provided for positioning, that is for centering the point or hairpin-type fllamentary cathode 65 with respect to the alined apertures of the electron optical system. This means-is of demountable character-permitting rapid replacement of the filament and also permits the use or diflerent types of filaments in connection with the other elements of the electron microscope. A further advantage oi the cathode supporting structure is the uniquearrangement by virtue of which certain portions of the structure not only serve as a mechanical support for the cathode but also serve as electrical conducting paths for the conduction of cathode heating current.

A metallic base I04 is sealed at one end to the vitreous part 32 and is provided with threads to engage threads at one end of the outer tubular conductor 34 to the base I04 which not only transmits power to the filament 66 but also serves to impress through the longitudinal conductor shown in Fig. 13 a negative potential on the fllament and the intermediate electrodes of electron lenses 30-32. Base I34 is constructed to ailord a flexible mechanical connection between the inner conductor 63 of the concentric hue and the base. A conductive connecting member III is mechanically supported by base I04 but is electrically insulated therefrom by means of a glass bead or seal I36 which rigidly supports member I05. At one end of member I66 we provide a flexible connection such as a coil spring I" attached to the inner conductor 63.

The base I04 is also provided with a threaded flange I68 which supports a metallic thimble I" provided with a shoulder I it which longitudinally positions the cathode positioning or centering means to be described immediately. This cathode centering means is flexibly connected to member I05 by means of a flexible connection such as a coil spring III and comprises an annular centering ring 2 having near the periphery thereof a restricted region H3 to obtain a substantial line engagement with shoulder I5Il. Ring II2 may be positioned by a suitable mechanical expedient such as a plurality of circumierentially which engages threads on the inner surface of thimble I66. As a means for insulating the shell oi thimble ill and the centering means from the conductor to the cathode; we empl y a ring- I I2 and .a cylindrical cathode base III! adapted to receive a rigid cathode stem I I6. One terminal I26 of the point cathode 66 is connected to stem III through a face plate I2I which is attached to stem H3, the latter constituting an electrical path tor the inner conductor 83 through coil spring III, connecting member I06 and coil spring Ill. The other terminal of the cathode I22 is connected to a metallic pin I23 supported by and electrically insulated from the face plate I2I by means of a glass bead I24.

As a means for facilitating ease of replacement of the cathode element 66 and to permit mechanical movement of the stem 3 and cathode 46 without disturbing the electrical connections to the cathode, we employ a flexible conductor such as a ribbon conductor I26 which extends into an opening I26 in the body of thimble I66. A screw .I 21 or any suitable mechanical expedient may be employed for retaining and removing the ribbon conductor I26 in opening I26. The electrical path from terminal I22 of cathode 66 comprises pin I23, conductor I26, thimble I03 and tubular conductor 64 of the concentric line. At the end of thimble I09 we provide a cap I26 which is removable or demountable and which is adapted to be supported by thimble lll9by a screw-threaded engagement at I29. This cap I23 is provided with an aperture I30 through which the electrons emerge upon acceleration due to the fleld produced by anode cup I3.

An alternative flexible connection for the cathode supporting structure is shown in Fig. 5 and corresponding elements have been assigned like reference numerals. Instead oi employing coiled springs as the flexible connection and extending one of the springs through member I I6, we may employ folded strips I3I and. I32 of resilient metal, respectively, connected to members H8, I06 and I06, 63.

The actuating mechanism for releasing the pressure on cover 66 of the valve 65 closing the specimen insertion chamber is shown in Fig. 6. Rod 61 which exerts pressure against cap 56 may be removed therefrom by movement of handle I I in a clockwise direction, causing the clockwise rotation of a pivoted actuating arm I33 which is pivoted at some point to the right of the figure and which is connected to rod 61 through a crank I34. connected to actuating arm I33 by means of a connecting rod I36 which is positioned to actuate valve 62, thereby admitting air to the insertion chamber and the main evacuation chamber so that the cover 66 may be removed.

In Fig. 7 there is diagrammatically illustrated, partially in perspective and partially in crosssection, the unitary electron lens assembly showing the plurality of spaced electrostatic lens systems 36, 3i and 32, and corresponding elements have been assigned like reference numerals.

Referring to the electron lens 32,.it will be noted that the intermediate electrode 49 thereof, which corresponds to the intermediate electrodes 31 of Member 63 which ac-tuates Valve 62 is also electron lenses and II, is supported by an insulator I36 which holds the intermediate electrode 49 firmly in position and alinement. Provision is made for the impression on the intermediate electrodes of each lens system or a potential corresponding to the cathode potential by means of the conductor shown in Fig. 13. This conductor extends longitudinally the electron gun and is placed within the cylinder 29 extending through apertures I31-I9I. These apertures are also finely machined or ground to have convex peripheries to minimize high electric fleld gradients.

The conductor is a sectionalized conductor to facilitate assembly and disassembly of the electron lens unit, and comprises a plurality of conductors I42, I43 and I99 which are welded or otherwise attached to the intermediate electrodes and are provided with recesses to receive telescoping intermediate members I95 and I99.

There is also provided within the electron lens assembly electrostatic shielding means spaced longitudinally between the sectionalized conductor and the electron path. This shielding means may comprise a plurality of planar metallic shielding members I91 and I98 which closely engage the inner surface of cylinder 29 and abut facing outer electrodes of the electron lenses 39-32 inclusive.

As means for facilitating assembly or the entire electron lens unit and for positioning the actuating rods 9395 in a position which is mechanically expedient and which will not deleteriously affect the operation of the electron lens system, we provide longitudinal grooves lying in cylinder 29 and adapted to receive and permit longitudinal movement of rods 99-95. Only one of these grooves, that is groove I99 which receives rod 93, is shown in Fig. 7. Each of rods 9395 is terminated in a foot I99 which engages heads 99 of the screw-thread devices 96- 98 shown in Fig. 3.

As a, means for locking or keying the electron lens assembly in the desired or fixed angular position within cylinder 29 there may be employed suitable means such as a tongue I5I constituting an integral part of or attached to the cylinder, and which may be adapted to engage one of th annular spacing rings such as ring 99 shown in Fig. 3.

The manipulator 96 is shown in detail in Fig. 8 and constitutes a system of multiple ways which are mutually adjustable or controllable to produce three independent movements at right angles to each other. That is, the manipulator 99 upon actuation by rods 9399 produces independent horizontal-transverse, vertical-transvers and longitudinal or axial motion. The transverse longitudinal and vertical motions are employed for scanning purposes and the axial motion is employed for focusing purposes, the latter being necessary in view of the fact that the focal length of the electrostatic lens system provided by the microscope is fixed by the geometry of the electrodes or lenses thereof.

The manipulator shown in Fig. 8 is provided with three actuating levers I52, I59 and I99, only two of which are shown in Fig. 8. The details of construction and operation of the manipulator will be explained in connection with Figs. 14-20 inclusive. As to the general features of operation, it may be said here that the specimen carrier '95 issupported at one end by an annular ring I55 constituting a part of the manipulators, and this ring by virtue of the afore- 12 mentioned ways moves the carrier 99 tor scanning and focusing purposes. The entire manipulator is supported by a face plate I99 which is maintained in the desired axial position by ongagement with the inner surface of cylinder 29.

Fig. 9 is a detailed cross-sectional view of the specimen carrier 99 which may be 01 the cartridge type comprising a retaining shell I91 having a longitudinal opening I99 through which the electron beam passes. and having a resilient holding or positioning member such as a plunger III also apertured. The plunger I99 may be biased by means or a compression spring I99. .The carrier I99 is adapted to receive a locket shown in Fig. 12 which holds a specimen, and the distance between the inner vertical surface of shell I91 and the face oi plunger I99 is such that a slight pressure must be exerted upon the locket to obtain an insertion and in order firmly to hold the locket during adjustment oi the manipulator 99.

In Fig. 10 there is illustrated the structure for supporting the beam limiting disk 99. structure comprises a face plate I9I which is supported from face plate I99 by means 01. a plurality of spaced supporting rods I92 and I63. Other rods, not shown, may also be employed to ail'ord a firm support. Molybdenum disk is held in position by a shroud ring I99, the latter of which is positioned by a plurality of thumbscrews I99 which may be adjusted to obtain the desired alinement of the aperture in disk itwith respect to the electron gun and the apertures of the electrodes constituting the electron lens system. Thumb-screws I99 are supported by an annular member I99 having a central aperturc within which shroud ring I99 and disk 9!: are positioned and is provided with a clamp I9". to control the axial displacement of these mem-- bers. Face plate I9I and annular member Iii: are provided with apertures I99 and I99 which. however, [or the purpose of construction and arrangement are not inalinement with the corresponding apertures I91--I9I in the disks or electrodes of the electron lens assembly or aperture I19 oi face plate I99 shown in Fig. 8. These latter mentioned apertures are provided for the P rpose of receiving the sectionalired horizontal conductor shown in Fig. 13 and which will be described in detail hereinafter.

A specimen holder is shown in Fig. 11 and arranged to be inserted through the specimen insertion chamber 99 shown in Hg. 3 and into slot 9| and includes a locket I1I comprising a recessed ring I12 adapted to receive a line removable mesh upon which the specimen is placed and a locking and retaining ring I19. Rings I12 and I19 are shown in cross-sectional detail in Fig. 12.

In Fig. 11 there is also shown a suitable specimen holder which comprises a stem I19 springbiased to the position illustrated with respect to a cylinder I19. Stem I19 telescopes cylinder I19 and the latter is attached to a curved holding plate I19 whereby spring I11, in the absence of pressure on knob I19, draws the stem I19 upwardly. The recessed disk I12 is flexibly connected to the stem I19 by suitable means such as a line link chain I19. The lower end oi stem I19 is provided with a curvature to engage the periphery of disk I12 upon depression 01' member I19, th'ereby firmly holding the locket in engagement for the P rp e of extending and positioning the locket in carrier 99 shown in P 8. 9.

In Fig. 18 there is shown a portion of'a longitudinal sectionalized conductor for impressing cathode potential on the intermediate electrodes of the various electron lenses 38-82 and corresponding elements have been assigned reference numerals employed in Fig. '7. Due to the fact that the apertures I68 and I68 of the beam limiting structure are not in alinement with apertures I81-I4I of the lens assembly and aperture I18 of the manipulator faceplate I88. we provide an elbow I88 which telescopes section I42 of the conductor and which is provided with a highly polished and finely ground cup I8l adapted to receive a resiliently mounted pin I82 constituting a part of section I88 and which in turn telescopes a second elbow section I84 which is in electrical contact with and received by thimble I88 of the cathode supporting structure shown in Fig. 4.

Referring now to the details of the specimen manipulator shown in Fig. 8, reference is here had to the exploded view of the manipulator shown in Fig. 14. The only elements there illustrated are the face plate and supporting ways. The manipulator per se is disclosed and claimed in U. S. Letters Patent No. 2,380,209, granted July 10, 1945, to Charles H. Bachrnan and Jacob Tschopp.

There is provided on face plate I86 a pair of vertical ways I85 and I86 having therein races I81, preferably of V-shaped configuration to support a plurality of friction reducing bearings such as miniature ball bearings I88. A vertical transversing member I88 is also provided with races I88 and I8I to engage the ball bearings I88.

Attached to the transversing member I88 there is provided a pair of horizontal ways I82 and I88 also comprising V-shaped races I84 and I88 adapted to receive miniature ball bearings I88. A horizontal transversing member I81 provided with suitable races I88 and I88 engages the ball bearings I88. A third set of ways. namely the horizontal or axial ways for focusing purposes comprising ways 288 and 28I, are supported by the horizontal transversing member I81 and also comprise V-shaped races 282 and 288 likewise provided with miniature ball bearings 284 for reducing friction. Ring I88 constitutes a part of an L-shaped member 285 provided with races to engage the ball bearings held by races 282 and 288 and is ultimately supported by ways 288 and 28l.

Appreciation of the manner of operation of the manipulator shown in detail may be obtained y referring to Figs. 15-20 inclusive. Figs. 15-20, inclusive, do not show all of the elements of the manipulator 4!. The associated groups I8, I8; I1, I8 and I8, 28 show only those elements deemed necessary in order to emphasize the various independent movements produceable by the manipulator, the complete perspective view being shown in Fig. 8. Figs. 15 and 16, which are front and side views respectively of the manipulator, when operated to produce horizontal transverse motion by actuation or lever I82, indicate that upon movement of lever I82 forward, that is away from the drawing, lever I82 actuates crank 286 (shown in Fig. 8) which in turn rotates connecting rod 281 to move cam 288, thereby exerting a pressure against the structure for transversing member I81, and thereby moving the horizontal transversing member I88 to the right. There is also provided means in connection with each of the movements for substantially eliminating or reducing back-lash which, in connection with the arrangements shown in Figs. 15

and 18, comprises a spring 288 which biases member I88 towards the left observed from the front view shown in Fig. 15.

Operation of the vertical transverse motion produced by the manipulator may be obtained by referring to Figs. 17 and 18 which are, respectively, front and side views showing additional structure for obtaining this motion. Lever I84 is connected to rod 2I8 which in turn operates a cam 2I I connected to the way I82.

Back-lash prevention means is also provided in connection with member 282 to bias this member downward and toward member 288, and may comprise a spring 2I2a. Forward motion, that is a motion to the left of lever I84 viewed from Fig. 18, raises ways I82 and I88 thereby raising ring I88 and the associated carrier 48.

The manner in which the focusing or longitudinal movement of ring I88 is obtained may be appreciated by referring to Figs. 18 and 20. Actuating lever I88 is connected to member 288 by means of a crank or connecting rod 2I8 and a earn 2, the latter of which engages member 288 to move it horizontally or longitudinally. The spring 2I2b serves to prevent back-lash,

The structure through which the actuating wheels, including illustrated wheels 1 and 8, for controlling the manipulator are joined to and operatively associatedwith the screw-thread devices may be more fully appreciated by referring to Fig. 21. Certain of the elements there illustrated have been described hereinbefore and corresponding elements have been assigned like reference numerals. The end plate or disk 86 shown at the left-hand part of Fig. 3 is presented in enlarged form in Fig. 21 and shows the manner in which the contact or connecting pins I8I extend through that disk affording suitable connections for a plurality of flexible cables 2I8, 2 I6 and 2I1 which are provided with adaptors 2I8 and which are connected to the knurled wheels 1, 8, and 8.

A viewing aperture mounted at the end of the horizontally disposed cover for the electron gun, and represented by the reference numeral 8 in Fig. 22, may be designed to be removable and comprises an eye-piece 2I8, a light or glass lens 228 supported by a cylinder I which is attached to eye-piece 2I8 and which is insertable within a holding cylinder 222. The light lens 228 is in ailnement with the fluorescent screen provided by disk 88 shown in Fig. 21 and serves to magnify the image appearing on this screen.

An arrangement is provided whereby in place of using a light magnifying lens a camera 228 may be supported at the end of the cover cap 8 as illustrated in Fig. 23. In such instances, the camera 228 which may also include a magnifying lens is provided with an adaptor 224 which is insertable within holding cylinder 222 and may also be provided with horizontal supporting ririembers 228 and a further supporting member Fig. 24 diagrammatically illustrates an electric circuit diagram which may be employed for the application of anode-cathode voltage to the electron microscope and for the control of the cathode heating current. The electron microscope which we provide is capable of being energized from any suitable source of alternating current such as that readily available at the usual commercial power outlets. For example, the system may be energized from an alternating current circuit 221 of commercial frequency. The main switch 22 shown in Fig. 1 may be connected in the position illustrated to control the application of voltage and current to the microscope as well as to control the application of power to the pumps is and I1 and centrifugal blower IS. A unidirectional voltage is produced by means of a rectifier circuit which may include a single unidirectional conducting device such as an electric discharge device 228 which is energized from a transformer 229. The voltage controlling means l3 may comprise means, such as a rheostat 233, connected in series relation with the primary windin thereof to control the magnitude of the unidirectional voltage applied to the microscope and may be actuated by knob 23 shown in Fig. 1.

One terminal of the secondary winding of trans-- former 229 is connected to ground. By virtue of the polarization of the discharge device 223, conductor 23l is maintained at a negative potential with respect to ground. Current limiting

resistances

232 and 233 may be connected in the positions illustrated not only to limit the current transmitted by discharge device 228 but also to limit the current supplied to the anode-cathode circuit of the electron microscope. In order to ffect the application of a substantially constant unidirectional voltage to the microscope, there may be provided suitable filtering means such as a capacitance 234 which is connected between ground and the common juncture of

resistances

232 and 233.

Transformer 235 is employed as a source of cathode heating current or filament current for the discharge device 228. Variation of the oathode heating current supplied to cathode 65 of the electron gun may be obtained by any suitable arrangement such as a transformer 236 controlled by means such as a rheostat 231, the latter of which may be actuated by switch or dial 2i shown in Fig. l.

Conductors 238 constitute means for supplying cathode heating current to the concentric line comprising inner conductor 83 and the tubular outer conductor 84 shown in Fig. 3. An adaptor (not shown) may be employed for connecting conductors 23i and 238 to the elements of the electron gun through a shielded cable 239 comprising a grounded metallic shield 240, conductors 238 which extend therethrough centrally, and a tubular conductor 24!, the latter of which is connected to the outer tubular conductor 8| of the concentric line.

In operation, the specimen or object to be investigated is placed upon a fine wire mesh which is inserted within the recess of ring I12 constituting one part of locket I1! shown in Fig. 12, and the looking or retaining ring I13 is employed to maintain the mesh firmly against the inner shoulder or ring I12. The holding plate H6 is grasped and the knob I18 is depressed, thereby extending the rigid stem I14 so that the curved lower portion thereof firmly engages the outer periphery of ring I12, thereby positively holding it. Cover 56 of valve 55 is removed and the specimen holder is extended through chamber 54 and slot i and the locket is inserted in specimen carrier 45, particularly between the inner vertical surface of shell I51 and the face of plunger I59, the latter serving to hold the locket firmly. The knob I18 is then released which permits retraction of the stem I13 upward into cylinder I15, which action renders the locket I1l free to be positioned b the carrier 45 and manipulator 46. The holder is then permitted to rest against the side or vertical wall of insertion chamber 54, the length of the holder being such that it does 16 not extend suflicientl upward to prohibit the closing or placement of the cover 53.

The valve 55 is then sealed by placing the transverse rod 51 in the groove provided in cover It, and handle ll of the valve is moved in a clockwise direction, viewed from Fig. 6, to apply sealing pressure to the cover.

Evacuation of the chamber enclosing the electron gun, the lens system and other associated elements is then initiated by the operation of handle or knob 23 shown in Figs. 1 and 24. After the pressure within the chamber has been reduoed to a suitable value, power may be applied to the electron microscope; that is, anode-cathode voltage may be applied to the electron gun by operation of knob or dial 20 and the filament current may be controlled by knob 2|, both of which are illustrated in Figs. 1 and 24. Of course, rheostat l3 permits adjustment of the voltage applied to the anode and cathode of the electron gun, and correspondingly controls the magnitude of the voltage impressed across the outer electrodes and the intermediate electrode of each of the electrostatic lenses 3032 The electron image formed at the specimen plane is magnified by the electrostatic objective lenses 3l|32 to produce an enlarged imag on the fluorescent screen 85. Where desired, the light optical magnifying lens shown in Fig. 22 may be employed to obtain further magnification.

Movement of the object for viewing and focusing purposes may be effected through operation of the knurled wheels 1-9 within the vicinity of the eye-piece 6 which, in turn, position heads 99 of the screw-thread devices 9$93 inclusive which are operatively connected to levers I52- I54 of manipulator 36 through rods 93-95.

The specimen may be removed in the following manner: Handle ll shown in Fig. 6 is raised effecting release of the pressure on cover 56 so that this cover may be removed. However, prior to the removal of the cover, as stated above, heated air is admitted to the insertion chamber 54 and to the main evacuated chamber through passage 60 by means of valve 62 which is actuated by arm I33, connecting rod I35 and member 63. As soon as the differential between atmospheric pressure and the pressure within the chamber is suflloiently reduced, cover 56 is then readily removable.

Referring to Figs. 3 and '1, which indicate in detail certain aspects of the electron lens assembly, we have found that a high degree of precision is obtained by spacing the

outer electrodes

33 and 33 at distances from the intermediate electrode 31 which are at least equal to or which approach the diameter d of the electron aperture of intermediate electrode 31. In addition, it has been found advantageous as concerns operation of the electron lens system as a whole, to make the distance s between center lines of each electron lens at least two times as great as the diameter d of the aperture of the intermediate electrode 31 or at least two times as great as the spacing it between the

outer electrodes

33 and 33 and the intermediate electrode 31. As concerns the relative diameters of the apertures in the

outer electrodes

33 and 33 and the aperture of the intermediate electrode, it has been found that the aperture of the inner electrode should have a diameter at least twice the value of the aperture diameters of the outer electrodes.

As a further matter, referring to Fig. '1 in conjunction with Fig. 3, we have found that in order to obtain the desired electrostatic shielding between the longitudinal sectionalized conductor and the electron beam. the longitudinal conductor should be spaced at a distance a: which is at least as great as the distance 7:. between the intermediate electrode I1 and the outer electrodes II and II.

Although in the illustrated embodiment of our invention we have shown the electrostatic lens of the type comprising an intermediate electrode having an aperture of larger diameter than the apertures in the outer electrodes, it will be appreciated that our invention is not limited to that particular type of electrostatic lens. It should be understood that the focal properties of a lens of this type are greatly determined by the distribution of potential along the axis and that the distribution of potential is readily determinable by the electrode positionand configuration. A large variety of electrode constructions and placements may be utilized for attainment of a desired potential distribution. The structure which we provide is, therefore, readily adaptable to a wide variety of electrostatic lens assemblies and configurations.

One of the important advantages afiorded by the electron microscope which we provide and which contributes in a large measureito' the desired precise operation thereof, by eliminating the eil'ect of extraneous fields, is the substantially continuous shielding of the electron beam provided by the structure. For example, beginning at the electron gun as shown in cross-sectional view in Fig. 3, the disk-like members 66 and 61 which constitute the magnetic shielding means and the transverse electrostatic shielding member 61' define a substantially field-free region through which the electron beam passes after being accelerated by the anode cup 13. It will be noted that the planar member 61' is interposed between the electron path and the longitudinal conductor. Furthermore, the specimen carrier 48 may be constructed of a suitable shielding material such as brass, thereby afiording the desired shielding of the specimen and the electron beam from the field due to the longitudinal conductor. As pointed out above in connection with Fig. 7, the'planar members I" and I48 also provide electrostatic shielding within the unitary electron lens assembly.

While we have shown and described our invention as applied to a particular device and as embodying various other devices, it will be obvious to those skilled in the art that changes and modifications may be made without departing from our invention, and we therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit I and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. In an electron microscope, the combination including an evacuated chamber, a unitary electron lens assembly adapted to be inserted within said chamber, a specimen carrier, means for manipulating said specimen carrier for scanning and focusing purposes, and means extending longitudinally of said chamber for actuating the last mentioned means for moving said carrier independently in three mutually perpendicular directi ns.

2. In an electron microscope, the combination including an evacuated chamber, an electron lens system comprising a metallic cylinder supported within said chamber and having therein electrostatic lenses, a specimen carrier, a manipulator tem, rods positioned within said chamber in iongitudinal grooves in the exterior surface of said cylinder for controlling said manipulator, and exdtgmally accessible actuating means for said ro 3. In an electron microscope, the combination including an elongated evacuated chamber, a unitary electron lens assembly adapted to be inserted in said chamber, an electron gun, a specimen carrier intermediate said electron lens system and said gun, means for adjusting the position of said specimen carrier comprising a manipulator within said chamber and susceptible of three mutually perpendicular directions of motion, and iongitudinally extending means positioned within said chamber for actuating said manipulator.

4. In an electron microscope, the combination including an evacuated chamber, an electron gun located at one end of said chamber, a unitary electron lens assembly inserted within said chamber and supporting a specimen carrier, a manipulator susceptible of positioning said carrier, actuating rods positioned within said chamber and extending longitudinally therethrough, a viewing screen at the other end of said chamber, and externally accessible means at said other end of said chamber for operating said rods.

5. In combination, an electron microscope having an evacuated chamber, a unitary electron lens assembly comprising a metallic cylinder having therein an electrostatic lens, a specimen carrier, a manipulator within said chamber for supporting said carrier and for moving the specimen carrier for scanning and focussing purposes, a viewing aperture at one end of said chamber, a plurality of actuating rods within said chamber positioned in longitudinal slots in the exterior surface of said cylinder and engaging said manipulator, and a plurality of actuating means within the vicinity of said aperture for moving said rods.

6. In an electron microscope, the combination including an evacuated chamber, an electron lens assembly comprising a metallic cylinder having therein an electrostatic'lens, a specimen carrier, a manipulator within said chamber for supporting said carrier, an actuating rod within said chamber extending longitudinally the axis of said cylinder and outside said cylinder for actuating said manipulator, a wall at one end of said chamber, and a screw-thread device supported by said end wall for actuating said rod.

7. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, an electron lens assembly comprising a second metallic cylinder having therein an electrostatic lens, a specimen carrier, a manipulator within said chamber for supporting said carrier and for moving it laterally, vertically and longitudinally, a plurality of actuating rods within said chamber extending longitudinally said lens assembly and engaging said manipulator, a wall sealing one end of the first mentioned cylinder and supporting a plurality of screw-thread devices for moving said rods, and sealing means for each of said screw-thread devices.

8. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, a unitary electron lens assembly comprising a plurality of electrostatic lenses, a specimen carrier supported at one end of said assembly, a manipulator for said carrier within said chamber,'a plurality of actuating rods within said chamber extending longitudinally said lens assembly and engaging said manipulator, a wall at one end of said cylinder for sealing that end and comprising a fluorescent viewing screen, a plurality of screw-thread devices positioned within said chamber about said screen for actuating said rods and each comprising a bellows-type member for sealing said device to said end wall, said devices being provided with pins extending through said wall, and externally accessible actuating means connected to said pins.

9. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, an electron lens assembly, a specimen carrier supported by said assembly, a specimen insertion chamber extending radially said cylinder, a. sealing structure for said insertion chambercomp: sing a removable cover, and means for supplying preconditioned air to the first mentioned chamber prior to the removal of said cover and valve means for controlling the last mentioned means.

10. In an electron microscopathe combination including an evacuated chamber, an electron lens assembly within said chamber, a specimen carrier, a radial specimen insertion chamber. a valve sealing structure for said insertion chamber comprising a removable cover, means for exerting pressure on said cover, a passage for admitting air at atmospheric pressure and a valve in said passage, and means interconnecting said pressure exerting means and said last mentioned valve for actuating said last mentioned valve prior to the release of the pressure on said cover.

11. In an electron microscope, the combination including a horizontally positioned evacuated chamber, an electron lens assembly within said chamber, a specimen carrier positioned at one end of said assembly, a radial specimen insertion chamber in alinement with said carrier, a sealing valve for said insertion chamber and comprising a cover adapted to seal said chamber, a passage for the admission of air to the first mentioned chamber and a valve within said passage, and actuating means for said cover and said last mentioned valve comprising a lever mechanism for actuating said last mentioned valve prior to the release 01' pressure upon said cover.

12. In an electron microscope, the combination including a horizontally positioned evacuated chamber, an electron lens assembly within said chamber, a specimen carrier positioned at one end of said assembly, a radial specimen insertion chamber in alinement with said carrier, a sealing valve for said insertion chamber and comprising a removable cover, a passage for the admission of air to the first mentioned chamber and a valve within said passage, an actuating means for the second mentioned valve, a bellowstype diaphragm for sealing said actuating member, and means for actuating said member prior to the release of pressure on said cover comprising a pivoted actuating arm and a manually operable handle.

'13. In an electron microscope, the combination including a horizontally disposed evacuated chamber defined by a metallic cylinder, an electron lens assembly within said cylinder, a specimen carrier at one end of said assembly, a radial specimen insertion chamber in alinement with said carrier, means including a valve sealing structure external to said cylinder for sealing said insertion chamber, and a specimen holder said insertion chamber.

14. In an electron microscope including an evacuated chamber, an electron lens system, an electron gun and a viewing screen, the combination comprising a'specimen carrier, evacuating means for said chamber, a specimen insertion chamber, sealing means for said insertion chamber, means for supplying preconditioned air-to said first mentioned chamber to prevent the condensation oi appreciable moisture upon the elements within said first mentioned chamber, and means for admitting the heated air prior to the opening of the insertion chamber.

15. In an electron microscope, the combination including an evacuated chamber, an electron lens assembly, a specimen carrier positioned at one end of said assembly, a specimen insertion chamber transverse to the longitudinal dimension of the first mentioned chamber, manipulating means for supporting and moving said carrier, and means for inserting a specimen through said insertion chamber into said carrier comprising a specimen holder having at one end thereof a locket, said manipulating means having a radial slot ior'guiding said locket to a desired position. i

16. In an electron microscope, the combination including an evacuated chamber, an electron lens assembly comprising a plurality oi disk-like electrodes constituting an electrostatic lens and having alined apertures, an electron gun for establishing an electron beam, a specimen carrier positioned adjacent said assembly, a manipulator for supporting said carrier, and means for inserting a specimen into said carrier comprisin a specimen holder having at one end thereof a relatively movabl locket, said manipulator having a radial slot for guiding said locket to a position of alignment with said apertures.

17. In an electron microscope, the combination including an evacuated chamber, an electron gun, a lens system comprising a plurality of apertured alined electrodes, a specimen carrier arranged to support a specimen within the path 01 the beam produced by said gun and to permit the passage or said beam therethrough, and a specimen holder for inserting a. specimen to be examined in the path 01 said beam comprising a rigid member, a locket adapted to be inserted in said carrier and flexible means between said locket and said rigid member.

18. In an electron microscope, the combination including an evacuated chamber, an electron gun, an electrostatic lens system comprising a plurality of apertured alined electrodes, a specimen carrier having therein a longitudinal opening to permit the passage therethrough oi the beam produced by said gun, and a specimen holder for inserting a specimen in the path of said beam comprising a rigid stem, a locket for supporting said specimen and adapted to be inserted in said carrier, a fiexible connection between said stem and said locket and means for extending said stem to engage said locket for insertion of said locket into said carrier and imretracting said stem after insertion into said carrier whereby said locket is free to be positioned.

19. In an electron microscope, the combination including an evacuated chamber, an electron gun in said chamber, an electrostatic lens system within said chamber and comprising a plurality of apertured and alined electrodes, a specimen carrier provided with a longitudinal opening to permit the passage of the electron beam produced by said gun therethrough and including a positioning ring, and means for positioning a specimen to be examined across said opening comprising a rigid stem, an apertured locket for supporting a specimen and a flexible connection between said locket and said stem. said stem being positionable to engage firmly said locket for insertion into said carrier against said positioning ring and for releasing said locket so that it is unrestricted in movement established by said carrier.

, CHARLES H. BACHMAN.

SIMON RAMO.

REFERENCES CITED record in the UNITED STATES PATENTS Number Name Date 2,272,843 Hillier Feb. 10, 1942 2,330,888 Hillier Oct. 5, 1943 2,200,095 Mal-ton May 7, 1940 2,266,218 Krause Dec. 16, 1941 2,380,209 Bachman et a1. July 10, 1945 2,284,710 Zworykin et a1 June 2, 1942 2,215,979 Schuchmann Sept, 24, 1940 2,264,209 Krause Nov. 25, 1941 2,058,914 Rudenberg Oct. 27, 1936 2,275,234 Ruedy Mar. 3, 1942 2,361,722 Von Ardenne Oct. 31, 1944 2,347,348 Young Apr. 25, 1944 OTHER REFERENCES Wilson, "a Vacuum-Tight Sliding Seal, vol. 12, pp. 91-93.

Zeit. fur Physik, vol. 115, Pp. 339-368, March