GENERAL SPECIFICATION NO. 263.

[Revised February 10, 1910.]

SERVICE FIELD GLASSES.

1. Preliminary.—This specification covers the design and construction of field glasses, types A and B, each having two powers as hereinafter specified.

2. Sample.—The bidder shall furnish with his proposal a sample of the glass which he will supply, and award will be made after comparison of the samples with models on file in the office of the Chief Signal Officer. The maker will be allowed to examine the model glasses in detail in the office of the Chief Signal Officer of the Army, Washington, D. C.

3. Inspection and test.—When the order under this specification is complete, the contractor will notify the Chief Signal Officer of the Army, who will cause an inspection to be made. It shall be the duty of the contractor to remedy any defects pointed out by the inspector, and the contractor will be held accountable for any imperfections which the inspector may have overlooked.

Fig. 34.—Field glasses, Types C and D.

The Chief Signal Officer of the Army reserves the right to inspect any or all processes of manufacture, and unsatisfactory material will be marked for rejection by the inspector before, during, or after assembly, as occasion may arise.

Each glass will be tested for power, field, definition, and light. Any glass which is not the equal of the sample and model in all respects will be rejected. The properties above enumerated will be tested as follows:

(a) Power: In testing for power the glass will be placed upon a firm support about the height of the eye and directed upon a range rod, accurately divided into divisions of 1 foot, with alternate divisions colored red and white, respectively. The rod should be placed approximately 100 feet from the glass in a good light and with strongly contrasted background.

The rod is observed through the glass with one eye and at the same time with the other eye unaided. An accurate comparison of the two images by means of the rod scale determines the magnifying power of the glass.

(b) Field: The field will be determined by the use of a transit or any other instrument adapted to the measurement of horizontal angles. The glass will be placed upon the telescope of the transit in such a way that the axes of collimation of the telescope and field glass barrels are parallel. The extreme limits of the field of view of the glass are marked in a convenient way and the horizontal angle of view accurately measured with the transit.

(c) Definition: In determining the definition of the glass expressed in units (seconds) a target will be provided with a number of lines one-tenth inch thick with one-tenth inch spaces between them drawn on a piece of heavy white paper.

At a certain distance this target will appear uniformly gray when viewed through the glass.

The inspector will gradually approach the target, focusing the glass until he reaches the most distant point from the target where the uniform field ceases and the black and white intervals appear distinct and defined.

Assume the distance thus found to be 20 yards and the thickness of the lines and intervals between them one-tenth inch. The circumference of a circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by 3.1416, or 45,240 tenths inches; but a circumference equals 360°, or (360 by 60 by 60) 1,296,000 seconds.

If, therefore, 45,240 tenths inches correspond to 1,296,000 seconds, then one-tenth inch equals 1,296,000 divided by 45,240, or 28.6 seconds. The definition is therefore 28.6 seconds, or practically half a minute.

The definition should be as follows:

For 6.5 power glass	30 seconds.
For 5.5 power glass	35 seconds.
For 4.5 power glass	40 seconds.
For 3.5 power glass	55 seconds.

(d) Light: The light of a field glass is expressed by a number which is the ratio of the amount of light which reaches the eye through the glass to the amount which enters the eye unaided. This comparison will be reached by means of the absorption apparatus furnished by the Signal Corps. This apparatus consists of two wedge-shaped vessels made of brass with glass windows in the sides, and are filled with a perfectly black liquid. The sky line is first viewed through the apparatus with the naked eye and the instrument adjusted to limit of visibility. The reading of the scale is then noted. The sky line is again observed, using the glass, but in other respects as before, and a second scale reading obtained. The ratio of these readings measure the illuminating power of the glass which must conform to the standard sample.

4. Service field glass, type A.—(a) This glass shall conform in general to the model, now on file in the office of the Chief Signal Officer at Washington. The arrangement for changing automatically from the low power to the high power, and vice versa, by the interposition of the plus lens at the proper distance in front of the eyepiece, must be strictly adhered to.

(b) The low power shall be approximately 3½ diameters and the high power shall be approximately 5½ diameters. The figure of merit given by multiplying the numbers of diameters power by the number of degrees of field will be considered in the examination of samples, along with the other properties of light, sharpness of definition, and general excellence.

(c) The tubes, frame, and metal fittings shall be of aluminum or an aluminum alloy, with the exception that such metal parts as in the opinion of the maker require greater strength may be made of brass.

Tubes shall be held firmly in the frame, single draw, the draw action to be through a bearing surface of at least five-eighths of an inch of best black felt, perfectly fitted so as to preserve perfect alignment.

The exterior metal parts, except where leather covered, must be given the best and most durable, lusterless black finish. The tubes and shades will be neatly covered with best quality tanned calfskin, the leather to be sewed on, and the seams to lie flat next to the focusing standard.

The interior of all parts to be painted a perfectly dead black.

The sunshades, when drawn out, shall project at least five-eighths of an inch and not over 1 inch beyond the edge of the cell.

The focusing screw and standard should follow closely that of the sample, except that the milled focusing disk should have a face as nearly one-half inch wide as possible and the milling should be sharper.

In addition to the diaphragm upon which the automatic lens is mounted, there shall be two diaphragms in each tube, so situated and so proportioned as to cut off all stray light and all internal reflections.

The crossbar supporting the draw tubes should be shaped and engraved exactly as found in the model.

(d) The lenses must be entirely free from mechanical defects, such as specks, air bubbles, etc.; must be free from interior strain, and must be ground from the best obtainable glass for the purpose, selected for general transparency, as colorless as possible, perfectly ground and polished, and accurately centered.

The object lenses shall be composite, achromatic, and well corrected for spherical aberration, with a clear aperture of at least 1½ inches, and not exceeding 1⁵/₈ inches. Bidders will state the number and shape of the pieces used to make up this lens.

The compound lenses may be either cemented together with Canada balsam, or left uncemented, as the maker may deem best for durability and optical performance, but if left uncemented the components shall have a permanent mark to indicate their proper positions in the cell.

The eyepieces shall consist of a single double concave lens having a clear aperture of not less than three-eighths of an inch and not more than one-half of an inch.

(e) The sling cord attached to eyes in the frame by means of brass snaps with black burned finish shall be round and braided from four strands of pliable tan leather, and shall have a diameter of at least one-eighth of an inch and not over one-sixth of an inch.

(f) The case and strap must be exactly like sample, and of No. 1 stock. Care must be taken to put in only compasses that are in perfect condition. The strap buckle must be of brass. The glass, when closed, must not exceed 4 inches in length, and the glass, case, cord, and strap, complete, must not exceed 25 ounces in weight.

(g) The frame shall be constructed with jointed bars for interpupillary adjustment.

5. Service field glass, type B.—(a) The requirements of part 4, service field glass, type A, of this specification, shall be followed in the design and construction of the type B glass in so far as applicable.

(b) Power: The lower power shall be approximately 4½ and the high 6½ diameters.

(d) Case: Case and carrying strap shall be furnished as required in part 4 of this specification.

(e) This glass shall be constructed with jointed bars for interpupillary adjustment.

(f) The sunshade, when drawn out, shall project not less than three-eighths of an inch and not more than 1 inch beyond the edge of the cell.

6. Marking.—Glasses furnished under this specification shall be marked on one barrel with the words "Signal Corps, U. S. Army," and on the other barrel "Serial No. ——." Serial numbers will be furnished with the order. If not furnished the contractor at the time the order is placed, the Disbursing Officer of the Signal Corps should be called upon for same, and the numbers and other marking placed on the glasses prior to the delivery of the order.

James Allen,
Brigadier-General,
Chief Signal Officer of the Army.

Signal Office,
Electric and Telegraph Division.

Transcriber's Notes:

Page 23, "porportions" changed to "proportions" (in proper proportions)

Page 106, "engineeer" changed to "engineer" (a French engineer)

Page 126, opening bracket added to subtitle ([Revised February 10, 1910.])