US2218270A

US2218270A - Microscopy

Info

Publication number: US2218270A
Application number: US229491A
Authority: US
Inventors: Homer C Snook
Original assignee: HOMER C SNOOK; MAY MCKEE SNOOK
Current assignee: HOMER C SNOOK; MAY MCKEE SNOOK
Priority date: 1935-11-25
Filing date: 1938-09-12
Publication date: 1940-10-15
Anticipated expiration: 1957-10-15

Description

Oct. 15, 1940. H. c. SNOOK MICROSCOPY 9 Sheets-Sheet 1 Original Filed Nov. 25, 1935 ATTORNEY.

H. C. SNOOK MICROSCOPY Oct. 15, 1940.

INVENTOR. Homer 6'. Sneak ATTORNEY.

H. C SNOOK MICROSCOPY Oct. 15, 1940.

9 Sheets-Sheet 3 Original Filed Nov. 25, 1935 INVENTOR. Hazzzer C. $200k H. C. SNOOK MICROSCOPY Oct. 15, 1940.

Original Filed Nov. 25, 1955 9 Sheets-Sheet 4 INVENTOR.

Homer 67511001! ATTORNEY H. C. SNOQK IICROSGOPY Oct. 15,1940.

Original Filed Nov. 25, 1935 9 Sh'eets-Sheet 5 INVENTOR.

Home) C'- Sneak M aL 6164 ATIDRNEY.

H. C. SNOOK MICROSCOPY Original Filed Nov. 25, 1935 9 Sheets-Sheet 6 INVENTOR. 7

ATTORNEY.

Oct. 15, 1940.v

H. C. SNOOK MICROSCOPY Original Filed Nov. 25, 1935 9 SheetsSheet 7 INVENTOR.

Homer aJ JzM/r ywhza fi z ATTORNEY.

1940- H. c SNOOK 2,218,270

MICROSCOPY Original Filed Nov. 25, 1935 9 Sheets-Sheet 8 Anystrms Ftyl ' INVENTOR.

Homer C. Sneak ATTORNEY.

Oct. 15, 1940; H c, N K 2,218,270

MICROSQOPY Original Filed Nov. 25, 1935 9 S heets-She et 9 IN VEN TOR.

ATTORNEY.

Patented Oct. 15, 1940 MICROSCOPY Homer 0. sneak, Summit, N. 'J., assignor to himself and May McKee Snook, his wife, as joint tenants Original application November 25, 1935, Serial No. 51,427. Divided and this application September 12, 1938, Serial No. 229,491

11 Claims.

My invention relates to microscopes and particularly to microscopes whose optical or imageproducing systems include reflectors or mirrors.

In accordance with one aspect of my invention, the mirrors are surfaced with material having low coeflicient of absorption and suitably high coefficient of reflection for wavelengths of radiation within the range of from about 2000 Angstrom to about 200 Angstroms.

Further in accordance with my invention, the objective mirror comprises annuli, whose reflecting surfaces are portions of diflerent ellipsoids having common conjugate foci, so arranged en echelon the radiations reflected therefrom are in proper phase relation to produce a real image. Further in accordance with my invention, a

a plurality of mirrors aflording diiferent magni- Y scale and with parts in section. of the primary flcations of an object are so mounted in fixed relative positions upon a common support rotatable to effect alignment of a selected mirror with the axis of the microscope, that, regardless of the mirror selected, the real image of the object is always in a flxed position.

My invention further resides in features of construction, combination and arrangement hereinafter described and claimed.

This application is a division of my application Serial No. 51,427, flied November 25, 1935, which is in part a continuation of my application Serial No. 10,431, flled March 11, 1985.

For an understanding of my invention and for illustration of apparatus embodying it, reference is to be had to the accompanying drawings in which:

Fig. 1 diagrammatically illustrates an imageproducing system for producing photo-micrographic or fluoro-microscopic images;

Fig. 2 is a side elevation of apparatus having an image-producing system of the type shown inFig. 1;

Fig. 3 is a plan view of the apparatus of Fig, 2; Fig. 4 is a detail view, in plan. on enlarged head of the apparatus shown in Fig. 2; a

Fig. 5 is a view in elevation, taken on line 5-4 of Fig. 4; partly in section and with parts omitted;

Fig. 8 is a side elevation of parts appearing in Fig. 5; Fig. '1 is a detail view, in elevation and on enlargeo scale, of the secondary head of the apparatus shown in Fig. 2;

Fig. 8 is a plan view of the mirror turret shown in Fig. 7;

Fig.0 9 is a detail view taken ;on line 0l of Fig. 1 i

Fig. 10 is a sectionalview taken on line "-0 ofFig.9;

Fig. 11 diagrammatically illustrates an arrangement for selectively using different sources of radiation in the apparatus shown in Figs. 2 and 3;

Fig. 12 diagrammatically illustrates a source of radiation specifically diflerent from that shown in Fig. 1; g

Fig. 13 diagrammatically illustrates an arrangement for reducing the overall length of an apparatus such as shown in Figs. .2 and 3;

Fig. 14 is a cross-sectional view of a preferred form of objective mirror;

Fig. 15 is a graph referred to in explanation of the image-producing system;

, Fig. 16 illustrates another modification for obtaining wide-angle irradiation of the object.

In the present microscopic practice. the maximum useful magnification is about 1,000 diameters, and the resolution about 0.11 micron. Al-

though higher magniflcations have been em-' obtained with the refracting type of microscope.

Although it is appreciated the resolution obtainable with a microscope increases as the wavelength of the light or radiation used is decreased, there are known no suitable refracting materials which are sufiiciently transparent to radiation much below 3,000 Angstroms. Radiation of these short wavelengths can be generated by electronic bombardment of various metals at various voltages or by condensed sparks and arcs in vacuo, but prior to my invention they have not been used to produce real images in either photomicrography or fluoro-microscopy.

To obtain substantially enhanced useful magniflcation and resolution far in excess of those possible with a retracting type of microscope now employed, I utilize an image-producing system whose components, as diagrammatically shown in Fig. 1, comprise a suitable source S of radiations within the range below 2,000 Angstroms, and an arrangement for selecting a narrow band of wavelengths and impressing it upon the object or specimen 0. Specifically, the monochromator or apparatus for providing radiations within a desired narrow band of wavelengths may comprise, as shown, a source of radiation 55, a primary condenser mirror C, a diffraction grating D and a secondary condensing mirror 1. The radiation, generated by the source S, which as shown may be a target T bombarded y electrons emitted by the cathode c, is directed y the primary condenser C upon the diffracon grating D which is viewed by the secondary ndensing mirror G1 which focuses the narrow 3 mi of radiation selected from the spectrum 1 reduced by grating D upon the object O. The radiation may also be provided by an electric spark or are between electrodes E, E of Fig. 12. By suitable selection of the material of the target, or of the electrodes, and of the electric voltage and wave form employed, a band of radiso ation including the desired narrow band of substantially monochromatic radiation can be obtained.

ihe selection of different narrow bands of radiction from the source S for irradiation of the object can be effected by varying the angular relation to each other of any one or more of the elements 8, D and C, preferably or most simply by adjustment of D, the diffraction grating.

The object O is at one conjugate focus of the mirror M, whose other conjugate focus is located in front of the second mirror Ml, preferably ellipsoidal, which views the real image of the object produced by mirror M and impresses a magnified real image upon the photographic or fluorescent plate P.

It is to be noted that the radiation received by the diffraction grating D, instead of emanating from a point source or a slit, as is customary in spectroscopic practice, is collected by the condensing reflector 0 through the large solid angle or cone of radiation 1'. The irradiation of the object is therefore substantially enhanced. Further, the diffraction grating D, as below stated, is surfaced with material selected to have a high coefllcient or reflection at the wave-lengths used. This also affords greater intensity of irradiation of the object at the wavelengths desired.

The reflecting surfaces of the mirrors M, Ml, C and Cl, and of grating D, and of any of the other mirrors herein referred to, are of a material which for the band of radiation used has an appreciable coefllcient of reflection and a low absorption coefficient; for example, the surfaces may be of one or more elementary metals such as aluminum, magnesium, beryllium, cobalt, silicon, sodium, caesium, lithium, potassium, nickel or rhodium or metal alloys which may be deposited in any suitable manner as by sputtering or evaporation. The support, backing or form for the reflecting surface should be of material having an insubstantial thermal coemclent of expansion, such as fused quartz, invar, pyrex, etc. The surface of the material of the backing of the mirror is ground, polished and figured after the manner employed in the production of optical mirrors. The metallic coating subsequently ap plied to the supporting surface may be polished and figured optically, if necessary or found desirable.

For the range between 2000 Angstroms and 200 Angstroms (one Angstrom equals .0001 micron) the effect of air absorption is so great as to preelude transmission of radiation through air at atmospheric pressure. Therefore within this range of wavelengths, or for any wavelengths for which occur refractive or other adverse effects of the air or gas between. elements of the imageproducing system, whether it employs mirrors as in Fig. l or is of a refracting type shown in a subsequent modification, the path of radiation is in a vacuum preferably high and of the order of 10- millimcters of mercury, or lesser pressures. As shown in Fig. l, the entire image-producing system may be enclosed in a housing I connected as by the pipe or conduit 2 to a suitable vacuum pump.

As it may be desirable, in some instances, to have adifferent degree of vacuum where the radiation is produced, there may be provided an inner casing la which closely fits the path of radiation in the monochromator to provide a gas path of substantial impedance between the source of radiation and the major portion of housing I which encloses the mirrors M, MI and plate P. Separate pumps may be connected to the

outlets

2 and 2a on opposite sides of the impedance.

The use of mirrors eliminates chromatic aberration, and therefore any narrow band of a wide range of wavelengths can be used to illuminate the object without loss of definition. It is usual practice to correct the lens system of a refractlng microscope for usually not more than three diflerent wave-lengths of visible light, and at all other wavelengths there is more or less chromatic aberration. The mirror system, therefore, not only obtains results possible with the known refractive systems at the wavelengths for which the refractive elements are sufllclently transparent, but has the features of advantage over the refracting type of microscope for still shorter wavelengths. With the lens or refractive system good definition or clarity of the image can be obtained only at the few long wavelengths for which the lens system. is corrected, whereas with a given mirror system good photomlcrographs are obtainable at a plurality of related wavelengths most of which are in the range of wavelengths shorter than visible light. The reflecting materials mentioned above are suitable for the range of wavelengths including visible light and extending substantially below 2000 Angstroms. Whereas, with the present types of refracting microscopes the maximum useful magnification is of the order of 1,000 diameters and the resolution obtainable is of the order of 0.11 micron, my method and system afford a wide range of useful magnifications, as to a maximum of upwards of 10,000 diameters and with resolutions better than 0.11 micron.

Figs. 2 and 3 illustrate in greater detail the construction of a microscope system of the type shown in Fig. 1. The tube 3 between the housing 4 for the source of radiation and the housing I for the diffraction grating, the tube 0 between housing 5 and the head Hi which contains the object, condensing and reflecting mirrors Cl and M, tube 1 which connects the primary head HI to secondary head H2 which contains the second mirror MI, and tube I between the secondary head H2 and the head HS which encloses a sensitized plate, such as a photographic or fluorescent plate P, are all interconnected to form a single rigid unit. because even slight relative movements ofanyofthepartsoftheimm-pmdudngeystem during exposure would blur the image and thereby prevent realization of the high degree ofresolution obtainable with the'apparatus. The

sitory disturbances as caused by railroad trains,

automobile trucks, and the like. By making the apparatus as rigid as possible and substantially isolating it from the effect of seismic and other disturbances, relative motion of the parts and consequent distortion of the image is rendered negligible.

Referring to Figs. 4, 5 and 6 which disclose in detail the internal construction of the primary head HI, the radiation from the refiecting mirror Cl is focused upon the object O which is carried by the adjustable support It, preferably a lazy tongs device mounted within the opening H in the object carrier l2. The object can be moved axially of the tube 1 by the lazy tongs l0 which are adjustable externally of the apparatus as by the knob l2, and is adjustable transversely of tube 1 as by the knob H, and can be moved angularly about the axis of the object carrier 12 by adjustment of the knob l5. These adjustments are made while the object is being observed through the view telescopes l6 and I1, while the object is illuminated either with visible light from the monochromator, as in Fig. 12, or by means of fluorescence of the object when irradiated by ultra-violet light, wavelengths shorter than 4,000 Angstroms, also supplied by the monochromator. jectis adjusted to proper position with respect to the cross hairs of the view telescopes l8 and I1, it is then known to be located at the correct conjugate focus of mirror M so that the real image will be located at the correct corresponding conjugate focus of mirror M properly to be viewed by the second mirror 'Ml of the head H2.

A plurality of mirrors M may be provided for attainment of different orders of magnification-.-

The mirrors M are mounted on a common rotatable support or turret l8 which is adjustable externally oi the apparatus, as by the handle is, to bring any selected mirror'to operative position; The mirrors M are so mounted that each, in turn, when brought into axial alignment with the tube 1 is at approximately the correct distance I from the object O; i. e., at the position which will produce a real image always at the same position within tube I.

As shown in Figs. 2 and 3, the head H2 is at the other end of tube 1. Its internal construction, as shown in Fig. 7, comprises a turret 20 for supporting a plurality of mirrors MI of different focal lengths each of which is at such distance from the position of the real image produced by the mirror M that when brought into axial alignment with tube 1 the other conjugate focus of mirror MI is on a photographic plate or fluorescent screen P at the other end of tube 8. The turret 20 is adjusted externally of the apparatus as by the handle 2|, thus allowing the operator. by various permutations of the mirrors M and MI to obtain different desired magnifications. For certain low magnifications the turret 20 is located to bring the optical fiat 22 axially of the tube I and the object is adjusted so that the focus of mirror M conjugate with the object is on the photographic or fluorescent plate located When the obthe photographic or fluorescent plate are fixed. The mirrors in use at a given time have fixed image-producing positions, and the various magnifications are obtained by selecting the desired combinations of mirrors. Each mirror is called upon to perform the single task of making a real image for but one set of conjugate foci. This fixation of conjugate foci is of advantage as the mirrors can be corrected for the positions at positions of the object, of the real image and of which each of them is used, avoiding errors which 4 would occur if different magnifications were sought by changing the relative distances of the mirrors and object.

To ensure that the distances between the various elements of the image-producing system and their alignment remain constant, at least during the time of use or exposure, the tubes 3, 8,1 and l and their cooperating parts should be of material having a low temperature coefficient of expansion. Alternatively, or in addition, the apparatus may be enclosed in a housing which is period of operation.

Since the tubes 3, 6, I and 8 are of fixed length, the maintenance of a vacuum, or a desired atmosphere, is facilitated since there is avoided any need of telescoping or sliding joints to change the lengths of these tubes. The selection of the different mirrors at the primary and secondary heads can be effected without any loss of vacuum or change in the gas pressure or composition in the image-producing system bythe features of construction now described.

Referring to Fig. 4, the shaft 21 of the turret l8 and the conical sealing extension 28 thereof are ground to fit the plate 29 of the head which is clamped to the head housing 30, the interposed seal or gasket member ll preventing leakage at this point. The bearing surfaces are preferably lubricated with an oil or grease having a very low vapor pressure, such as Apiezon oil or grease, or N-dibutyl phthalate, or butyl-benzyl phthalate.

The insertion and withdrawal of the object carrier is accomplished with minimum loss of vacuum or gas, because movement of the carrier to the position affording access to the object moves a solid portion of the carrier to block communication between the tube and the outer atmosphere. Specifically, the cylindrical object carrier i 2 is ground to fit the tubular casing 32 which is interposed between the tube I and the primary head. These bearing surfaces are also lubricated with a lubricant having a low vapor pressure.

To withdrawthe object the slide is moved down- For this position the enclosure ducing air into this space. The slide then may be fully retracted to bring the object space within the opening ii so that it is external to the apparatus, allowing insertion or replacement of the object O. The slide may be then returned to the position bringing the upper edge of the opening ll somewhat below the edge 33 of the casing so that the object space is again in com munication with the port 34 allowing the object space either to be evacuated or filled with gas at a desired pressure corresponding to composition and pressures within the image-producing system. The slide is then moved to bring the object to proper position axially in tube 1 for making of another photomicrograph or visually observable image.

The construction of the secondary head is generally similar to that of the primary head. Referring to Fig. '7, the shaft portion 35 for the turret 20 and the conical extension 36 is ground to fit the cooperating portions of the plate 31 which closes the secondary head. The joint is preferably lubricated by one of the low pressure lubricants above mentioned. As indicated, the

extend to tube 3.

. time I.

gasket 38 is clamped between plate 31 and the cooperating flange of the head housing 3!. The window 40 in plate 31, which permits use of the view telescope 24, as above described, is alsosuitably sealed in the plate. The several openings ll, sealed by the plugs 42, are to permit insertion of suitable tools for adjusting the several mirrors Ml when initially installed. The different combinations of mirrors in the primary and secondary heads afiord desired different degrees of magnification, and the movement of the handles 1 I9 and 2| to effect a desired combination also brings the mirrors to such positions that the real images produced are at substantially correct positions.

The construction for permitting removal and insertion of the photographic or fluorescent plate P in the head H3 without loss of vacuum or gas is in general similar to that used in the primary head for allowing removal and insertion of the object 0.

Referring to Figs. 9 and 10 the cylindrical plate carrier 43 is ground to fit the opening 44 extending through the casing 45 which is provided with an opening 46 in alignment with When the plate carrier is in the retracted position shown by full line in Pig. 9, the photographic or fluorescent plate P can be removed from or placed upon the plate holder 41 which is preferably carried by a lazy tongs arrangement 48 The carrier 43 may be moved to the extended position shown by dotted line in Fig. 9, thereby bringing the plate P in its holder central with the axis of tube 8, but with the carrier 43 in a position which is so angularly dis- I placed from the position shown in Fig. 10 that port so. The valve u is then in position to efiect connection to a vacuum pump connected to the pipe 52, or, if desired, the passage Il may After the recess ll is exhausted or filled with suitable gas, depending upon the condition under which the image-producing system is to be ,used, the carrier 0 is then rotated to bring the plate holder to the position shown in FigIlO. To remove the plate, the reverse sequence of operations is performed. First, the holder 43 is rotated to bring the space 4! into communication with port 50. The valve Ii is then operated to effect communication of this space with atmosphere, and then the holder II can be withdrawn bringing the solid portion to the right of the holder, as viewed in Pig. 9, to block the end of tube i. The exposed plate may then be replaced by an unexposed plate.

This construction is also useful in oscillographs of the type using a photographic plate to record the path of a cathode-ray beam.

With the plate in position to receive the image, it may be uncovered by moving the shutteractuating knob 54, and is of course recovered before removal of the plate carrier by the same knob. The shutter and its actuating mechanism are now shown as any of various known arrangements may be used. The desired position of the plate axially of tube I can be obtained by adjustment of knob which is connected to the lazy tongs, preferably through a suitably calibrated reduction mechanism.

As indicated diagrammatically in Fig. 11, there may be disposed within the head 56 of the apparatus shown in Figs. 2 and 3 several sources of radiation for producing different wavelengths. The handle 51 external to the head 50 can be adjusted to bring a selected one of the sources in front of the condensing reflector C. One of the sources may be an incandescent lamp, an electric are between metallic electrodes, a disruptive spark discharge between metallic electrodes, as in Fig 12, or other source of visible light, and the other sources (Fig. 1) may be for producing radiation having a wavelength substantially shorter than Visible light.

Assuming that a photo-micrograph of a particular object is to be made, the desired source of radiation is brought before the condensing mirror, as above described, and the selected narrow band of wavelengths is brought to focus at the position of the image by adjustment of the monochromator. The adjustments may be made with the object in position when visible light is employed, or when the object will visibly fiuoresce at the wavelengths used. Adjustments may also be made by use of a calibrated fluorescent screen in the position of the object which is observed during adjustment of the monochromator to bring the desired narrow band of wavelengths to the position which the object will occupy. The monochromator may be calibrated, and one or more of the elements adjustable, the scales cooperating with the adjustable elements being calibrated so that when the parts are in predetermined positions the desired narrow band of wavelengths will be focused upon the object.

with the source of radiation deenergised the photographic or fluorescent plate can now be inserted and uncovered. as above described. With both the object and plate in position, the source of radiation is then energised for a predetermined length of time; the plate is then covered and removed from the apparatus without loss of vacuum. It is desirable that a series of photomicrographs be taken with the plates at slightly different positions axially of tube 0 because the focus, particularly for radiation of short wavelens hl. is sharper than can be determined visually. It may also be of advantage to take a series of 'photomicrographs at somewhat different wavelengths to obtain enhancement of contrast between diverse constituents of a heterogenous surface, as in metallography. It is to be understood, of course, that my invention is not limited to metallography, but comprehends bacteriology, histology. botany, hi-

ology, crystallography, and, in general, the determination of the fine microscopic structure of all materials.

In many instances, it is desirable to hold the temperature of the object under observation at a magnitude higher or lower than room temperature. This control can be effected by means included in or associated with the object carrier. For example, the object carrier may support adjacent the object a tube provided for circulation of a heating fluid, or a refrigerating fluid such as liquid air.

The physical dimensions of the apparatus will be determined largely by the magnifications desired and the construction of the optical system. For example, the apparatus shown in Figs. 2 or 3 may be of the order of 20 to 30 feet long. In some cases it may be desirable to reduce the overall length of the apparatus by folding the optical paths, as indicated in Fig. 13, by use of plane reflecting mirrors between some of the optical elements hitherto described. For example, as shown in Fig. 13, the plane reflecting mirror Pa may be interposed between the source of radiation and the diffraction grating D; the plane reflecting mirror Pl may intervene between the diffraction grating and the secondary condensing mirror CI and the plane reflecting mirrors P2, P3, P4 may be disposed at other points. in the optical path. 4

The entire system, as in the modification above described, is preferably maintained in a vacuum or suitable gaseous atmosphere. For microscopy at the wavelengths below 2,000 Angstroms the surfaces of the plane mirrors, as well as others of the system, should be of material having insubstantial absorption coefiicient and appreciable coefficient of reflection.

A suitable and preferred construction for the objective mirror M of the modifications of Figs. 1 to 13 is shown in cross-section in Fig. 14. The curved reflecting surface or vertex mirror V has a circular periphery and in the particular objective mirror shown extends about 5 on each side of the axis A. The reflecting surface V, concave toward F, is a portion of an ellipsoid whose first conjugate focus is at F, three inches from the vertex of mirror M, and whose second conjugate focus is thirty feet from the vertex. The reflecting surfaces e-e9 are annular and are portions of different ellipsoids which have the same common conjugate foci; F, the object focus, is three inches from the vertex of mirror M and Fl, the image focus, is thirty feet from the vertex of mirror M. For the conjugate foci F and Fl, no spherical aberration is produced by the ellipsoidal reflecting surfaces V, e-e9, and under the conditions of use spherical aberration is negligible in the produced images.

For sharp images free of spherical aberration, the object and image fields should not be greater than about Since the chord of at a radius of three inches is 0.0261 inch, whereas the size of the object under investigationis usually of the order of 0.001 inch to 0.002 inch, the size of the object fleld is well within the limit of tolerance. Furthermore, the image field of mirror M, for an object fleld of .001 inch diameter, has a diameterof 0.120 which, located at Fl, thirty feet from the vertex of mirror M, has a diameter which is a small fraction of the chord of and, therefore, thesize of the image field is also well within the limit of tolerance. Moreover, the image fleld possesses negligible spherical distortion not only longitudinally at Fl but also laterally over the angular extent of the image field.

As a condition precedent for negligible coma, each annular zone of the echelon objective mirror M should produce magniflcationsfrom the different extremes of its own surface that differ from each other by a negligible amount. In the mirror specifically shown in Fig. 14, the flrst focal distance for each annulus is 3"-* -0.0625 and the maximum difference in the magnifications is which is negligibly small.

The variation in magnification due to different distances from the axis of various parts of the object is caused by a variation in radial object distance from F of i002". This variation superimposed upon the focal distance produces a total maximum variation of :0.0645" giving i2.15% as the total percentage change of the magnifications. Coma is, therefor, negligible.

, Coma may still further be reduced by making the width of the annular zones of the echelon mirror angularlysmaller with respect to F. Each annular element may be made to correspond to "F numbers of any suitable relative aperture In the mirror of Fig. 14 the vertex mirror and two of the reflecting annuli are each in angular width, corresponding to F/5.737+, and each of the remaining reflecting annuli' is 5 in angular width and corresponds to F/ 1.46 for each annulus.

As previously stated, the reflecting surfaces of the objective mirror are, for wavelengths shorter than the'wavelengths of visible light, of a material, which for the band of radiation used, has an appreciable coeflicient of reflection and low coefllcient of absorption. The paraxial surfaces PA of the echelon mirror may, if desired, be of non-reflecting material to avoid diffusion of the radiation from the object.

The mirror MI is ellipsoidal with its conjugate foci f==l2 inches and fl =30 feet (at the plate P). Because the mirror is ellipsoidal, it is free from spherical aberration at its foci. The object for mirror MI is the real image produced by mirror M and which, as above stated, is about 0.0120" in diameter. At focus the real image subtends an arc of about V2 towards mirror MI, and at fl, the real image produced by mirror Ml on the plate is about 3.6" in diameter, which subtends an arc of about /2" towards mirror Ml. Therefore, the longitudinal and lateral spherical aberration with respect to mirror Ml is negligible. I

To obtain negligible spherical aberration, the relative apertures of the mirror elements should not be too great. This condition is satisfied by mirror M which, as above stated, has a relative aperture for three elements near the axis correspending to I /5337+, and for the remaining elements a relative aperture corresponding to i /11.46. The relative aperture of mirror Ml also satisfies this condition; since fl=12", the effective aperture is 0.3125 and the total aperture is 0.75". Therefore,

=F/38.5 and the image and object flelds only relatively small angular llOll'l the mirror axis in each case, asm, which is produced by sagittal, or nonmeri 12.1 rays, is negligible.

When the echelon objective mirror makes real images from objects illuminated by white light, destructive interference is produced at the real image of certain wavelengths, while, with other wavelengths which form the luminous tmage, there is cumulative lnterierence. The eflect of the dosructive interference is to decrease the brightness of the image. 1

If the microscope is to be used with monochromatic radiation, in addition to the foregoing conditions, the mirror M must be constructed to meet the requirement that the various rays of radiation from F which are reflected by all the elemental areas of all the annull, and of the vertex mirror shall arrive at Fl in phase with each other; otherwise, the destructive interference at Fl may destroy the image.

Since the vertex mirror and each of the annull are parts of ellipsoids, all of the rays reflected by each of the individual elements of any one ellipsold arrive at Fl in phase with each other, so that the requirement is met, considering any two adjacent annuli, when the rays reflected from the anterior edge of the posterior annulus are in phase with the rays reflected from the posterior edge of the anterior annulus.

This condition may be fulfilled whether or not the radial distance, or the paraxial distance between the adjacent edges of adjoining annuli is equal to a whole or integral number of wavelengths. Ihese radial and par-axial distances may each be some different integral number plus or minus different fractions of a wavelength provided that the focaldistanccs, I, for the two adjacent edges are in proper relation to those fractional numbers to ensure that the radiation from the edges are in phase at the posterior edge of the anterior annulus.

This result maybe obtained by polishing, or by removal of material from each annulus (or by addition of material) after its plate has been ccto fringes in the monochromatic lightfmmrtbat may be reflected by restricted mnes at the two adjacent edges of adjoining annuli.

For convenience, visible radiation is used in testing and adjusting the paraxlal distances between the adjacent edges of the reflecting annull; for example, 6003.039 A., one of the spectrum lines of iron, or 5438.47 A., one of the spectrum lines of cadmium. Assuming, for example, that the adjustment has been made at 6003.039 A., it is also correct for wavelengths 3001.519 11., 2001.013 A., 1500.759 A., 1200.608 A., 1000.506 A., 857.577 A., 750.380 A.. 667.004 A., etc; 1. e., the wavelengths whose relation to the chosen wavelength can be expressed by whole numbers. At any of these wavelengths. the cumulative interference at Fl will there produce a real image. Similarly, if the adjustment is made at 6438.47 A., images are produced at wavelengths of 3219.235 11., 2146.156 A., 1609.617 L, 1287.694 11., 1073.078 A., 919.78 A., etc.

The annular reflecting surfaces of the echelon mirror M are employed to increase the effective numerical aperture. of the mirror as an objective.

N. A. (numerical aperture) =11 sin 14 where n=1 (approx) for a vacuum sin u==sin angular aperture The objective mirror specifically shown in Fig. 14 has an effective numerical aperture of about 0.8; other suitable numerical apertures are Table A Total apctum In general, as appears from Fig. 15, the larger the numerical aperture, the greater the resolving power for a given wavelength. However, the increase in numerical aperture is attended with increased difficulty in satisfying the other conditions above discussed.

The limit of resolution with the best refractor type of microscope at present known is shown by the cross :c, Fig. 15. At a wavelength of 2750 1A., the smallest object that can be resolved has a diameter of 0.11 micron. Shorter wavelengths cannot be used because of the opacity or absorption of the quartx-mlorlte lens system at the shorter wavelengths. The limit of resolution with another high-grade refractor type microscope is shown by cross Y, Fig. 15. At a wavelength of 3650 A., the shortest wavelength used, the smallest obiect that can be resolved has a diameter of 0.140 micron. fllortcr wavelengths cannot be used because of the opacity or absorption of the glass lens system at shorter wavelengths.

with my reflector type of microscope, the limit of resolution is greatly extended. as shown by the curves of Fig. 16, which show the molutlons obtainable with the mirrors of Table A, for a range of wavelengths from about M00 Angstrom 0 about 200 Angstrom.

As above stated. high minim P wer in um less without correspondingly high resolving power. with any microscope having an optical system capable 02 working at a numerical apcrture (N. A3: oi the limit not by dim-action to obtain mtogrcphic rclolutton la the some n the wavelength; that is, the dlamctrr of tb equals the wavelength e" a we e; "i"- Limit of photographic resolum o :ion wfishsa numerical aperv") to me o Acute eye Average eye Poor eye At wool-0.2 microns- At 1000A.=0.1 microns At 500A.-=0.06 microns. At300A.-0.03 microns. at 200 0.02 microns 3, 810

The mirror system which I have specifically described afl'ords a magnification of 3600 diameters which, as appears from the table above, is suitably high to render visible the smallest object that can be resolved at wavelengths approaching 200 Angstroms.

For certain classes of work, for example, in metallurgy, the diameters of the specimen containing the object may be large, for example, of the order of 0.5 inch. In such case, the optical diameter of the object is limited to about 0.001 or 0.002 by a suitable stop coated with material which is substantially non-reflecting at the wavelength used. Preferably, in such cases, wide angle illumination of the object is used. .A suitable arrangement is shown diagrammatically in Fig. 16. The radiation from the monochromator is reflected by the conical mirror Cia onto the reflecting surface of the ring mirror Clb which, in turn, transmits the radiation to the object O. The stop Sh prevents the radiation from impinging upon the remainder of the specimen S. The object O is at one conjugate focus of the objective mirror M. The reflecting surfaces of conical mirror Cia and ring mirror Clb are of material which, for the wavelengths used, has

low absorptive power and suitably high reflective power. The remainder of the image-producing system may be the same as above described.

What I claim is:

l. A microscope comprising a rotatable support for a plurality of mirrors of different focal 5 lengths mounted in fixed positions thereon,

means for supporting an object to be magnified in the optical system of said microscope and adi justable for focusing of the object, and means for rotating said rotatable support to include a selected one of said mirrors in the optical system of the microscope for magnification of said object by the selected mirror.

2. A microscope comprising rotatable supports having relatively fixed axes of rotation and each carrying mirrors of diflerent focal lengths, means for supporting an object to be magnified in the optical system ofsaid microscope and adjustable for focusing of the object, and means for rotating said supports to include in the op- 3. A reflector type microscope having a miror comprising annuli whose reflecting surfaces are portions of diiferent ellipsoids having common conjugate foci and are of material having a low coeflicient of absorption and high coeflicient of reflection for wavelengths within the range of from about 2000 to about 200 Angstroms.

4. An objective mirror for a reflector type microscope comprising annuli whose reflecting surfaces are portions of diiferent ellipsoids having common conjugate foci, and which are so arranged en echelon, that the radiation from the anterior edge of each posterior annulus is in phase with the radiation from the posterior edge of the adjoining anterior annulus.

5.'A microscope comprising a pair of rotatable supports each carrying a plurality of mirrors, means for supporting-an object to be magnified in the optical system of said microscope, and

.means for varying the magnification of said object comprising means for rotating said supports to effect alignment of a selected mirror on one of 'said supports with a selected mirror on the other support.

6. A microscope comprising means for supporting an object to be magnified in the optical system of said microscope, a rotatable support,

'and a plurality of mirrors of different focal lengths so disposed on said support that by its rotation any one of them may be selected to magnify said object, and so mounted that irrespective of the mirror selected the real image of the object produced is always in the same position.

'7. A reflector type microscope having a mirror comprising annuli whose reflecting surfaces consist of material having low coefficient of absorption and suitably high coeflicient of reflection for wavelengths within the range of from about 2000 to about 200 Angstroms, are portions of different ellipsoids having common conjugate foci, and are so arranged .en echelon that the radiation from the anterior edge of each annulus is in phase with the radiation from the posterior edge of the adjoining anterior annulus.

8. A microscope comprising 8, housing, 8, 10-

tatable support within said housing, a plurality of mirrors of different focal lengths arranged on said support about its axis of rotation, structure for supporting within said housing an object to be magnified by the optical system of said microscope,oblect-focusing means operable externally of said housing to adjust the position of said structure, and means operable externally of said housing to rotate said rotatable support selectively to include one of said mirrors in the optical system of the microscope for magnification thereby of said object.

9. A microscope comprising a housing, rotatable supports spaced within said housing, a plurality of mirrors of dlflerent focal lengths arranged on each of said supports about the axis of rotation thereof, means for supporting an object within said housing, and means operable externally of said housing to rotate said supports selectively to include in the optical system of the microscope a desired mirror on each of said supports for magnification thereby of said object.

10. A reflector type microscope including a mirror having a numerical aperture insuring resolutions better than 0.11 micron at wavelengths within the range of from about 2000 to 200 Angstroms and which comprises annuli having reflecting surfaces which are portions of different ellipsoids having common conjugate foci and which are of material having low coefficient of absorption and high coeflicient of reflection for wavelengths within said range.

11. A reflector type microscope including a mirror having a numerical aperture insuring resolutions better than 0.11 micron at wavelengths within the range of from about 2000 to 200 Angstrorns and which comprises annuli whose reflecting surfaces are of material having low coemcient of absorption and high coefllcient of reflection {or wavelengths within said range,

which are portions of different ellipsoids having common conjugate Ioci, and which are so disposed en echelon that radiation from the anterior edge of each annulus is in phase with the radiation from the posterior edge of the adjoining anterior annulus.

HOIWER C. SNOOK.