When held as shown in [figure 32] with the tubes drawn out about 1 inch to secure proper focus, the glass has a power of about 5.6 diameters, and a field of about 5.4 degrees.
Fig. 32—Signal Corps field glass, Type A.
If the glass is turned into the position shown in figure 33, the small plus lenses, just in front of the eye pieces, drop automatically into position and reduce the power to 3.8 diameters, and increase the field to 8.3 degrees. This position requires a different adjustment, the tubes being drawn out about one-third of an inch to get the proper focus. It will be observed in the illustrations that the rear bar of the frame is not only lettered to indicate which power is being used, but the bar itself is shaped with a hump on one side, and hollowed on the other. When the hump is up, the low power is in use. This is to facilitate adjustment in the dark.
The action of the small automatic lenses is free and positive. Neither the eyepieces nor the sections containing the small lenses should be unscrewed, except in case of necessity, and then not by unskilled hands.
Fig. 33.—Signal Corps field glass, Type A.