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.