No advice or fixed rule can be stated as to what constitutes the most suitable characteristics of a field glass. No single field glass can furnish maximum results under all conditions on account of varying conditions of the atmosphere.
A high-power glass is unsuitable for use at night, hazy atmosphere, or for use of a mounted man where the glass can not be rested against a firm support. A low-power glass with large object lens to permit as much light as possible is a necessary condition for use at night. The double power glass which is issued as a part of the visual signaling outfits was designed for the military service as a compromise for conflicting conditions.
A brief description of the field glasses issued by the Signal Corps, together with the cost of the same, is given below.
Type A:
This glass is the current result of the efforts of the Signal Corps to provide a field glass that will meet the greatest variety of conditions, and insure efficient service to the greatest number of military observers. It is really two glasses in one—a day glass of medium power, and a night glass of low power.
Fig. 31.—Type A. Showing the field glass and case with sling cord, shoulder straps, belt loops, and compass
It is to be clearly understood that while this glass is considered superior for moderate ranges, it does not replace, under special conditions, for long ranges, either the porro prism glass or the telescope.