US1019412A

US1019412A - Single-observer range-finder.

Info

Publication number: US1019412A
Application number: US37000307A
Authority: US
Inventors: Archibald Barr; William Stroud
Original assignee: Individual
Current assignee: Individual
Priority date: 1907-04-24
Filing date: 1907-04-24
Publication date: 1912-03-05
Anticipated expiration: 1929-03-05

Description

A. BARR & W. STROUD. SINGLE OBSERVER RANGE FINDER.

APPLIGATION I'ILED APR.24, 1907.

Patented Mar. 5, 1912.

3 SHEETS-SHEET 1.

I CT

\QMH. M1717 HQ n m a Q2441 5km). m

A. BARR & W. STROUD.

SINGLE OBSERVER RANGE FINDER.

APPLICATION FILED APR. 24,1907.

Patented Mar. 5, 1912.

l,Ol9,12.

3 SHEETS-SHEET 2.

A.- BARR & W. STROUD. SINGLE OBSERVER RANGE FINDER.

APPLICATION FILED APR.24, 1907.

Patented Mar. 5, 1912.

3 SHEETS-SHEET 3.

ARGHIBALD BARR, OF GLASGOWySCOTLAND, AND WILLIAM STROUD, OF

' ENGLAND.

STA ES PATENT OFFICE.

LEEDS,

- Specification of Letters Patent;

Patented a125, 191-2.

Application filed llpri124, 1 907. Serial No. 370,063;

To all whom it may concern: I Be it known that we, ARoHInALn BARR;

of Glasgow, in the county of the City of i Glasgow, Scotland, and WILLIAM Srrnomn:

5 of Leeds, in the'county of York, England, have invented certain new and useful Improvements in Single Observer Range- Finders, of which the following is a specification. I r 1 This invention relates to improvements in single observer rangefinders of the fixedj base typeccomprising a reflector at each end of the base length-oi. the instrument bywhich the beams entering at the two ends are reflected inward at approximately right angles toward the center of the base with two objectives through which the beams of light respectively pass on their wayto eye piece reflectors provided at the center of the 5 base where the beams are directed outward E at approximately right angles to the base,

- so that observations may be made on the well known coincidence principle of action In instruments of this type it will be recog- ,5 nized that two complete telescopes are in; Volved which extend in coaxial alinement2 from the center to formthe instrument, and

it is necessary to explain, that in the class of 1 instrument to which this invention refers, the objectives in the telescopes, located be- I tween the end reflectors and the eye pieces, are situated at an appreciable distance from their respective end reflectors. In instru-- ments of this type the refraction requisite for operation has hitherto been efiected in the region between the objective and the eyepiece of one of the telescopes, a condition which restricts the range of observation 3 'of the instrument inasmuch as beams of; light coming from objects at ditferent'dis tances will in some cases only artially enter'the objectives,'an'd it is the'o ject of this invention to effect the refraction of the beams within a; sphere adjacent to one of the end reflectors or in the sphere adjacent to the two end reflectors of the telescopes, in order to produce i-improved instruments of the type in question ,capable of operation over a wide range, and, further, to provide;

means for producing. definite deviations of one or both 'of. the beams, in conjunction with the provision I of ap ropriate scales adapted for use with such evices, in order to enable a large motion of the indicating I device to be made for a given changeof I junction with the means for indicating the degree of actuation of the adjustable retations of these prisms in the process of deminimum range observable will be large range while enabling the instrument to work over a wide series of ranges.

In order to produce the deviation of one or both beams for operation according to this invention two adjustable rotatable re fracting prisms of small and approximately equal angular deviations are provided, and the essential feature is that neither of the rotatingprisms must be at an extended distance from one of the end reflectors. This essential condition may be accom-' plished by placing both prisms near one end reflector orby placing onenear' each end refiector, and again they may be placed near the end. reflectors either toward the outside or toward the inside of the instrument. Further it is an essential condition that the rotating prisms in whatever positions they may be placed will, as viewed from the distant object, appear to rotateduring'the op.- eration of taking a range, by equal amounts in opposite directions. In order to produce a definite deviation of the beams, if required, a subsidiary retracting prism (or prisms) may be provided adapted to be 'inserted at will into one (or both) of the beams in conjunction with the provision of ascale or series of scales appropriate to the definite deviation produced, for use in confra'cting prisms used in the operation of the instrument. v

According to our invention achromatic adj ustable retracting prisms are preferably used These prisms are mounted so as to be capable of rotation about axes perpendicular; or approximately so, to their own planes, and means are provided for producing equal rotermining a range. A scale is provided to indicate the rotation of the prisms which may be graduated so as to indicate directly the range of the target; and in order to obviate the necessity of using delicate micrometric' mechanism for measuring the 1'01 tation of the adjustable prismsit may in some cases be. advisable to use prisms of very small retracting angle. In this case the (especially with a very long base) and we may provide a subsidiary retracting prism P or P capable of being inserted in the path of one of the beams (preferablynear the end of the instrumenth so as to increase the td- 110 Arangement in detail for effecting the reaccordance with our present invention. In-

this figure A A are two reflecting prisms, which may be of anyv suitable type, the "pentagonal type "being indicated. V B and B are two objectives. C is a. system of re- 10 prisms P and P (preferably with one P having double the angle of refraction of P) we can utilize 4: scalesl(l) without P and P (2) with P alone (3) with P alone, (4) with both P and P In order that the nature of our invention performed may be more clearly and fully understood we shall now describe some examples of construction embodying this invention with reference to the accompanying drawings and to the letters and marks thereon.

In the various figures the same letters are used to indicate the sameor like parts when these occur. v

Figure 1 1s a diagrammatic representatio'n, plan, of one form of rangefinder' -constructed 1n accordancewlth'our present Fig. 1 is a front view of the invention.

Figs.2 and "3 show one arinstrument.

quired rotation of the operating prisms in which Fig. 2 is a section on line 2-2 of Fig.

. 1 3, and Fig. 3, a section on line 33 of Fig. 35

2. Fig. 4 shows one arrangement of the scale upon which the ranges are read. Figs. A 5, 6, 7, 8 and 9 are illustrative diagrams refrred to below in reference to the nature of the scale. Figs. 10, 11, 12,'and' 13illustrate the use of a supplementary prism (or .'prisms)- to give more open scales, as described below. Figs. 14, 15 and 16 are illustrative diagrams showing modifications in the arrangement of parts of theinstrument. Fig.1 isa diagrammatic representation of one form of instrument constructed in ilectors or prisms which we designateeyepiece prisms, designed to turn the beams outward through an eyepiece D, to enable the images to be' viewed by an operator in the manner common to this type of instrument. E and E are two retracting prisms, (preferably achromatic prisms) arranged to give equal and small angles of deflection to rays traversing them, and situated in this case in the paths of the beams as these come toward the reflectors A and A Theprisms E and E are capable of rotation about axes F and F perpendicular, or approximately so, to the planes of the prisms. Gearand the manner in which the same may be shown at E and E Fig. l, the-partial images of an object at (say) 1,000 yards are seen in coincidence. A rotation of the prisms in opposite directions so as to bring the thin edges toward the upper or under) side will cause the beams of light from a distant ,object to come into coincidence in the eyepiece field. The images seen in the eyepiece field will at the same time appear to move upward (or downward) but this does not matter since the object can be kept continuously in view'by a suitable rotation of the whole instrument about its longitudinal center line. A scale operated in connection with the gearing which eflects the rotations of the prisms E and E may be so graduated as to indicate, for any position of. the retracting prisms E and E the distance of an object the partial images of which are in coincidence for that posit-ion of the refracting prism. The reflectors A and A the eyepiece D, the mountings for the prisms E and E and the gearfor-operating these maybe conveniently supported in or from a tube G, while the objectives B and B and the'eyep'iece prisms C may -be carried on a short inner tube H, so supported within the tube G as not to be distorted by forces applied to the latter during use.

A convenient type of gearing for effecting the'requi'red rotation of the prisms E and E is illustrated in Figs. 1 2 and 3. Fig. 1 is an elevation of the whole instrument as seen" from the front. Fig. 2 is a front elevation, and Fig. 3 a side view (each partly in section) showing the compound prism E and the gear operating it.

In Figs. 2 and 3, E is shown mounted in a holder J formed externally (wholly or partially) as a worm wheel K which is caused to rotate by" the revolution of the worm L mounted on a shaft M The prism holder Jt is supported on a carrier N attached to,the outer tube G of the instrument The shaft M is connected to a working head (seen in Fig. l which may be conveniently placed for operation by .the right hand ofthe observer. A similar gearing is provided for. the prism E except that while one of the worms is right handed the other is left handed. Letters referringto the corresponding parts at the right hand end of the instrument have the aflix 2 thusJ M etc. The. scale on which the ranges are indicated may conveniently be engraved on the prism holder J or on the corresponding downward at an angle of 45 when viewing an object on the same level as the rangefinder.

In Fig. 4, Q is a drum of celluloid or other translucent material, having the scale S cut on its outer face, and illuminated by transmitted light from a conical reflector R which receives light through a suitable window T the scale S then revolves ,during'the operation of the instrument, past an index pointer Y. The-drum Q, is mounted on a holder U supported from thetube G, Fig. 4, by

a carrier V, and the holder U has attached to it, or formed upon it, a worm wheel W into which aworm X gears, the worm X being mounted upon, or driven from the shaft M which operates the worms L and L above referred to. The drum Q, may be caused to revolve less than one revolution for the whole range of operation of the instrument, in which case the scale may be graduated circularly upon it. In the arrangement shown the scaleis viewed through an eyepiece D suitably placed for use by the left eye of the observer. while his right eye is opposite the eyepiece D through which the image of the object is viewed.

.The action of the retracting prisms and the nature of the scale will be understood by reference to the diagramsFigs. 5, 6, 7 and 8.

Fig. 5 is a plan of the optical parts at the left hand end of the instrument. Suppose the prism A to be so constructed that a ray entering it is deviated through a right angle .as shown in Fig. 5, and that the ray 1. comes toward the prism A at right angles to the length of the instrument, so that after re-v flection in A it passes along the center line of the instrument to the eyepiece prisms, as

' shown by the course 2. If the retracting prism is in the position shown in Fig. 5 with the thin edge e directed outward, the entering ray which would take the course 1, 2 would approach the instrument horizontally from a near object as shown by the course 3.

.55 The distance ofthe object from which the ray 3, I and the corresponding ray entering the instrument .at the right hand end come, will be the minimum range that the instrument is designed to observe. Suppose, for example, that this range is 1,000

- yards. If now the prism is rotated through an angle of 90? so that the thin edge comes to the underside the entering ray that would follow the course 1, 2, will not be deviated in 5 the horizontalpl'ane, and its plan will be indicated by the dotted lines 4 in the figure. As a similar action takes place simultaneously at the other end of the instrument, the rays entering through E and E will be parallel, and come from an infinitely distant object. (The rays in this case would come from an object at a higher angular altitude than that from which the ray 3 came, but this does not affect the use, of the instrument, as the whole instrument can be suitably directed in altitude-to observe on an object at any angular altitude.)

Fig. 6 represents an elevation of the prisx'n E and of the rays shown in plan-in Fig. 5. (For clearness the prism E is shown in this illustration as being rectangular-in outline though in practice it may conveniently be made circular in outline as shown in Fig. 2). Suppose a screen erected in'front f E as shown at c in Fig. 5, the rays 3, 4, etc. would pierce this screen at points 3, 4, etc. in Fig. 6, such points lying on a circle. described about the point in which the ray 1 (produced outward) would pierce the screen. Such points on the screen might thus form a scale on which the dis tance of the objects viewed could be indicated. Thus in the example chosen (Fig. 6)

4 would correspond to an infinite distance, (represented in the figure by the symbol while 3 would correspond to a distance of 1,000 yards. When the object is at 2,000 yards, the ray coming to E would bisect the angle between 3 and 4 in Fig. 5 (very approximately) as shown by the course 5. The corresponding point on the screen would be at 5 as shown in Fig. 6. It will thus be'seen that, if a scale graduated in reciprocal manner be set out along the line 13 in Fig. 6, and the points projected upward to the circle there shown, the scale. on the screen would be as indicated on Fig. 6. i The angle 3, 1, 5, Fig. 6, is thus the angle throu h which the iriism E (and simultaneous y the prism would have to be rotated in order to pass from a range of 1,000

yards to one of 2,000 yards and the angle 5, 1, 4, the angle through which rotation would have to be made in passing from a range of 2,000 to one of infinity.

It will be observed that the angle on the scale from 1,000 yards to 2,000 yards is great and the angles between say 000 and 8,000 1 somewhat less than 90 as shown in Fig. 7

in which the angle between the rays 6 and 7 is less than a right angle by the angle subtended by half the base length at a range say strument is designed to indicate.

double of the minimum range which the 'in- Suppose as before that the instrument is to work between the ranges infinity and 1,000 yards. Thus the prisms A and A each set out an angle less than a right angle by the angle represented in circularmeasure by the fraction (or more precisely an angle represented trigonbmetrically by tan 3, where B represents the base length in yards, but

the difference between these values "is in' practical cases usually inappreciable. Now

let the prisms E and E have angles of de viation somewhat greater than the difference. (Fig. 7) Then when the prism between the angle between 6 and 7 and a right angle. E is turned with'its thin edge downward (or upward) the rays traversing it will not be deviated in plan and consequently this position of the prism E (with the ,prism E correspondingly placed) would correspond If now the prismbe turnedwith its edge 6 vertical and out-- to a range of 2,000 yards.

ward as shown in Fig. 7, the range indicated would be less than 1,000 yards, so that the indication 1,000 yards is reached when the prism is rotated through an angle less than 90, say, an angle of 60 froin' the position in which the edge is horizontal, and the indication infinity is reached when the prism is rotated through an equal angle inf the opposite direction. Fig. 8 illustrates the type of scale thus obtained, the points on the circle being the vertical projections. thereon of a reciprocal scale set outalong the horizontal line. as indicated by the numbers there shown. It is in no way essential that the center point of the rotation used should correspond to a range of double of the minimum range. This has only been' chosen as an example. The. scale may beopened out still farther at high ranges by suitable alteration in'the angles set out by the prisms A and A and the angles'of deviation of the prisms E and E as is shown for example in'Fig. 9. Again it is not es-' sential that the scale should read up to in- Y finity though it is convenient to arrange it I so, in order that tests and adjustments may be made by observations on the moon or on a star.

In some cases (in order to have a larger motion of the rotating prisms and a longer scale than can, be conveniently gotten in the arrangement above described for giving maximum and minimum ranges, or for other reasons) we adopt the following ar-- rangement: Suppose that it is desired to be able to lnd cate ranges from 1,000 yards to infinity, andyet to have a comparatively large rotation. of the refracting prisms, and

. a long scale, for ranges from 2,000 yards to infinity. We may construct the instrument with the angles set out by A and A and the angles of deviation of the'prisms E and and E be rotated, a series of ranges from- 2,000 yards downward will be indicated, and an additional scale may be provided to indicate these ranges. To prevent errors being made by reading the wrong scale provision may be made for bringing the sec- 0nd scale into view only when the additional prism is in action and the first scale into view only when the prism is out of action. In Fig. 10, let the ray 9 represent a ray coming from an infinitely distant ob- 7 Q jet and ray 10 one coming from an object at 2,000 yards. If now an additional prism P of a suitable angle (say about double the angle of deviation of E be inserted between E and A as shown in Fig. 11, the prism E will have to be rotated into the dotted position, (or some position near it) in order to indicate a range of 2,000 yards,

and on rotation into the position shown by full lines, theincoming ray will have such a direction as is shown by 11, coming from a point at about 15000 yards range.

If it were desired to indicate ranges to 500 yards (still retaining the same scale for ranges between infinity and 2,000 yards) a second prismP (or say double the angle of P might be inserted between' E and r provided and exhibited when the two prisms are in place. Then, with neither P nor P in action, ranges from say rinfinity to 2,000 yards would be indicated, with I inserted, ranges from 2,000 to 1,000 with P alone ranges from 1,000 to 667' yards, and

with P and Pf ranges from 667 to 500.

It is not essential to the use of such additional prisms that these angles of deviation should bear any special. relations to the angles of deviation of prisms E and E and they may be conveniently so arranged as to give series of ranges which over-lap somewhat. Nor is it essential that the prisms should be'inserted between the reflecting prism A and the retracting prism E. .They might be placed in front of preferably in a corresponding position betweenE and A and another scale prism E, or between reflector A and the ObJGQtlVG E Fig. 1. Again the instrument."

may obviously be designed, adopting the same principle, to read the; lower ranges when the pr sms are not inserted, and the higher ranges when the prisms are intions required in the cases above described,

or again the instrument may be designed-to read medium ranges without the prisms and higher and lower ranges with the prisms inserted.

One arrangement for readily inserting and removing the prisms, and suitably exhibiting the appropriate scale, is illustrated in Figs. 12 and 13. In Fig. 12 prisms P and P are shown mounted in holders f and f which are suitably guided to prevent rotations of the prisms.

by slides 71 and k arranged for operation from the outside of the tube G. Fig. 13 shows a scale drum Q of the type illustrated in Fig. 4 in more detail, provided with an index Y capable of motion in the direction of the axis of the scale drum Q, in the uides j 7' attached to the tube Gr. Thein ex piece Y is attached to a double ended lever the ends K and K of which are so connected to the rods 9 and 9 Fig. 12 that when the prism P Fig. 12 is inserted into the beam of li ht passing between E and A the end K of the lever is raised by an amount that causes the index Y to rise from the scale S to the Scale S Similarly when P is inserted the end K is raised by such an amount that the index rises from S to S and when both P and P are inserted the index Y rises from the scale S to the scale S.

It is to be understood that we do not confine ourselves to the use of optical squares of the pentagonal prism type at the ends of the instrument. Optical squares of other suitable forms may be adopted or optical squares constructed of two mirrors may be substituted, or single reflectors may be used, if due provision is made toprevent errors arising from the bending of the frame on which they are mounted.

The arrangements of the parts may be further modified in various ways :Thus. instead of having one of the rotating refracting prisms at each end of the instrument, both may be placed at one end, as shown diagrammatically in Fig. 14. In this are rangement provision is made for imparting opposite rotary motions to the prlsms E and E Again the prisms E and E need not be placed in front of the end reflectors as shown in Fig. 1. They may be placed near the end reflectors but one between each end reflector and the corresponding objective as indicated in Fig. 15. In this case the prisms should be rotated in the same direction. Again both prisms may be placed near one end reflector and between it and the corresponding objective and have opposite rotations.

Attached to the holders f and f are rods 9 and g operated When both refractingiprisms are placed in one of the beams, the additional prism (or prisms) P and]? Figs. 11 and 12 may be arranged to be inserted into either beam or into the two beams as may be preferred.

When the scale is fixed toone of the rotating prism carriers we may still arrange that it can be viewed by the left eye, while the right eye is used to observe the coincidence, by inserting suitable lenses and reflectors to convey an image of the scale to a second eyepiece, placed'at about 2% to the left of the eyepiece D, Fig. '1. rangement is shown diagrammatically in Fig. 16 in which S is the scale attached to and rotating with the prism carrier, J; I, Z21 are reflecting prisms; 172, 19311 ob ec-' tive; and D the left eyepiece. The arrangement of prisms and lens (or lenses) may be modified to suit the requirements in various forms of instruments constructed in accordance with our present invention.

a reflector at each end of the base and twoobjectives through which the beams entering at the two ends respectively pass and does not refer to instruments using one objective.

only.

Claims: 1. A single observer rangefinder compris- .ing two telescopes, parts of which extend in co-axial a-linement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation each dISPOSQd near an One such arend reflector, means. for rotating the prisms,

and a graduated scale moved in accordance with the prisms, for the purposesset forth.

2. A single observer rangefinder comprisingtwo telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an ap preciable distance from their respective end reflectors, two ,refractin prisms of equal angle of deviation each dlsposed outside and near an end reflector, means for rotating the prisms, and a graduated scale moved in accordance with the prisms, for the purposes set forth.

3. A single observer rangefinder comprising two telescopes, parts-of which extend in co-axial alinement from the center 0 the instrument two end reflectors, the obj tives of the telescopes being situated at n appreciable distance from their respective end angle of deviation disposed respectively near the said end reflectors, means for rotating the prisms, and a graduated scale moved in accordance with the prisms, for the purposes set forth.

"reflectors, twofrefracting prisms of equal 4. A single observer rangefinder comprising two telescopes, parts of which ext-end in" co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation disposed respectively outside and near the said endreflectors, means for imparting equal and opposite rotational motions to the prisms, and a graduated scale moved in accordance with the prisms, for the purposes set forth.

. 5. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two retracting prisms of equal angle of deviation each disposed near an end reflector, means for rotating the prisms, a. range indicating scale moved in accordance with the prisms, an index, a subsidiary prism that can be introduced at will into one of the beams and a subsidiary scale to be used when the subsidiary prism is, inserted for the purpose set forth.

6. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation each disposed outside and near an end reflector, means for rotating the prisms, .a range indicating scale moved in accordance with the prisms, an index, a subsidiary prism that can be introduced at will into one of the beams and a subsidiary scale to be used when the subsidiary prism is inserted for the purpose set forth.

7. A single observer rangefinder compris ingtwo telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation disposed respectively near the said end reflectors, means for rotating the prisms, arange indicating scale moved in accordance with the prlsms, an in-' dex, a subsidiary prism that can be introduced at will into one of the beams and a subsidiary scale to be used when the subsidiary prism is inserted for the purpose set forth.

8. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation disposed respectively outside and near the said end reflectors, means for imparting equal and oppositerotational motions to the prisms, a range indicating scale moved in accordance with the prisms, an index, a subsidiary prism that can be introduced at will into one of the beams and a subsidiary scale to be used when the subsidiary prism is inserted for the purpose set forth.

9. A single observer rangefinder comprising two telescopes, parts of which extend in co-axialalinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two .refracting prisms of equal angle of deviation each disposed near an end reflector, means for rotating the prisms, a range indicating scale moved in accordance with the prisms, an index, subsidiary prisms that can be inserted into the optical system and subsidiary scales to be used when the subsidiary prisms are inserted, for the purpose set forth.

10. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation each disposed outside and near an end reflector, means for rotating the prisms, a range in- .dicating scale moved in accordance with the prisms, an index, subsidiary prisms that can be inserted into the optical system and subsidiary scales to be used when the subsidiary prisms are inserted, for the purpose set forth.

11. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation disposed respectively near the said end reflectors, means for rotating the prisms, a range indicating scale moved in accordance with the prisms, an index, subsidiary prisms that can be inserted into the optical system and subsidiary scales to .be used when the subsidiary prisms are inserted, for the purpose set forth.

12. A single observer rangefinder comprising two telescopes, parts of which extend in co-axial alinement from the center of the instrument, two end reflectors, the objectives of the telescopes being situated at an appreciable distance from their respective end reflectors, two refracting prisms of equal angle of deviation disposed respectively outside and near the said end refiectors, means for imparting equal and 0p posite rotational motion to the prisms, a range indicating scale moved in accordance with the prisms, an index, subsidiary prisms that can be inserted into the optical system and subsidiary scales to be used when the subsidiary prisms are inserted, 1@ for the purpose set forth.

In Witness whereof we have hereunto set our hands in the presence of two witnesses.

ARGHIBALD BARR. WILLIAM STROUD. Witnesses:

JOHN LIDDLE, JOHN T. LIDDLE.