FM 23-45 Basic Field Manual
27
(1)
Interpupillary distance.—
To adjust the glass so that the eyepieces are the
same distance apart as the pupils of the observer’s eye, point the glass at the sky and
open or close the hinged joint until the field of view ceases to be two overlapping circles
and appears to be one sharply defined circle, then note the reading on the scale, which
indicates the spacing of the observers eyes. The similar setting of any other field glass
will then accommodate his eyes.
(2)
Focus of the eyepiece
.—Look through the glasses, both eyes open, at an object
several hundred yards away. Place the hand over the front of one telescope and screw
the eyepiece of the other in or out until the object is sharply defined. Repeat this
operation for the other eye, then not the reading on each diopter scale. A similar
reading on any other field glass will accommodate the same eyes.
d. Operation
.—
(1) In using the glass it should be held in both hands, lightly pressed to the eyes so
as to keep the relation with the eyes constant but not so as to transmit tremors of the
body. The bent thumbs may cover the corners of the eyes to exclude light except that
which enters the glass through the lenses. When possible, it is best to use a rest for the
glass or to rest the elbows on some solid object.
(2) The mil scales are seen when looking through the glass, and by superimposing
them on any objects the horizontal and vertical angles between those objects may be
read.
(3) The inverted sight leaf scale is used to secure range settings on sharply defined
auxiliary aiming points when the target is not clear enough for direct aiming.
e. Care
.—The field glass is a rugged, serviceable instrument but it should not be
abused or roughly handles. Care should be taken not to scratch or mar the lenses.
