were to come. In recent years the Carl 
Zeiss Works at Jena invented and 
patented a new eyepiece or ocular, 
which considerably extended the field 
of view obtained with previous models 
of prism binoculars. 
HIS wide-angle field is a most val- 
uable asset to a binocular, as it not 
only enables the eyes to locate objects 
much more quickly, but makes it a 
simple matter to follow swiftly moving 
objects, such as birds in flight, deer 
on the run, or high-powered motor cars 
tearing around the race track. In fact, 
even the viewing of a distant landscape 
of a lake and mountain, a more 
satisfactory picture may be en- 
joyed in one glance than would 
be obtained by alternate obser- 
vation of part of either moun- 
tain or lake. 
I have dwelt at length on the 
subject of field of view and have 
endeavored to explain its impor- 
tant bearing on a good binocu- 
lar. There ‘is, however, another 
valuable feature to consider and 
that is illumination or “light- 
gathering power.” It does not 
matter how great the magnifica- 
tion of your glass may be; it 
does not matter how wide the 
angle of view obtained—both 
these highly valuable qualities 
will be rendered practically use- 
less under poor light condition 
if your glass does not possess 
sufficient light-gathering power 
clearly to indicate detail in the 
object viewed. The amount of 
light-gathering power depends 
primarily on the size or diam- 
eter of the objective lens. Ob- 
viously the larger the diameter 
the more light will be admitted. 
The factor of magnification, 
however, determines the amount 
of light carried to. the ocular, 
and, the size of the objective be- 
ing constant, the increase of 
magnification decreases the light-gath- 
ering power. 
ee you will hold a prism binocular with 
the object toward some illuminating 
body, and look into the ocular from a 
distance of 8 or 10 inches from the eye, 
you will notice a small circle of light, 
which is called the “exit pupil.” For 
the purpose of identification and com- 
parison, light-gathering power is ex- 
pressed in numbers, these numbers 
representing the square of. the exit 
pupil. Thus a binocular having an exit 
pupil of 3 mm. has a light-gathering 
power of 9. An exit pupil of 4 mm. has 
a light-gathering power of 16, one of 
5 mm. 25 and so on. 
In any properly constructed glass 
each factor bears a definite relation 
to other factors, so that knowing one 
of them, others can be easily deter- 
mined. For example, if it is not con- 
venient to measure the exit pupil, light- 
gathering power may be computed by 
dividing the diameter of the objective 
by the known magnification and squar- 
ing the result, or we can determine un- 
known magnification by dividing the 
diameter of the objective by the exit 
pupil. 
Prism binoculars are generally desig- 
nated in accordance with magnification 
and diameter of objective lens. Thus 
a glass of 6 magnifications having a 
30 mm. objective is called a 6 x 30, 

Using the binocular in the field 
one having 8 magnifications and an 
objective of 40 mm. is an 8 x 40. The 
term “magnification” is an obvious one 
and is merely the ratio between the 
size of an object as it appears through 
the glasses and its size when seen with 
the unaided eye. Bearing this in mind 
one will experience little or no difficulty 
in selecting a glass of proper magni- 
fying power. The greatest distance at 
which objects can be viewed to advan- 
tage will not only depend upon mag- 
nification, but atmospheric conditions, 
locations and altitude. The almost uni- 
versal temptation to own a glass of 
high magnification can be readily un- 
dersood, but for general use a glass of 
6, 7 or 8 magnification will give the 
most satisfactory results. Then again, 
we must remember that with increase 
of magnification, we decrease light- 
gathering power and generally also 
field of view—two extremely important 
qualities which should not, except under 
extraordinary conditions, be sacrified 
for magnification. 
OR military or engineering use it 
may be desirable to use a field- 
glass for measuring distances, and for 
this purpose a graticule called “mil- 
scale” or “distance scale” may be in- 
corporated in the glass. This scale con- 
sists of a glass disk engraved with fine 
vertical lines, each space being equiva- 
lent to one-thousandth part of the 
focal length of the objective lens. 
In order to read the scale, one 
of two definite facts must be 
known—either the size of the 
object or its distance from the 
observer. Knowing object size 
we can determine the distance 
and, conversely, knowing the dis- 
tance we can tell the size of the 
object. 
For finding distance, we mul- 
tiply the known size of the ob- 
ject by one thousand and divide 
the product by the number of 
spaces covered on the scale, and 
divide the product by one thou- 
sand. 
In an early part of this story 
I mentioned that a simple tele- 
scope consists of an objective 
and an ocular. Now, in order 
that you may understand the 
focusing systems of binoculars 
I shall amplify this statement 
to the extent of showing the 
variation of the image focus 
produced by the objective. Prob- 
ably all of you are familiar with 
the camera, in which it is nec- 
essary to move the lens farther 
from the ground glass to photo- 
graph a close object and nearer 
to the ground glass for distant 
objects. At the proper focal dis- 
tance a sharp image will appear 
on the ground glass, plate, film or what- 
ever the intercepting medium may be. 
While the telescope objective acts very 
much like the photographic lens in that 
the position of the sharp image varies 
in accordance with the distance of the 
object, the resemblance in the two sys- 
tems ends there. 
ARS photographic lens projects this 
image on a screen where it is visible 
to the eye, but there being no such 
screen in the telescope, the image pro- 
duced is invisible, and can only be dis- 
cerned by aid of the collecting lens 
which “picks up” the image, and the 
ocular with which it is viewed. As, 
however, the point of sharpness of the 
objective image varies with the distance 
(Continued on page 182) 
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