40 THREE-DIMENSIONAL PHOTOGRAPHY

Thus, when a magnifying glass is used, we may assume that we

see a very large image at an infinite distance or we see a smaller

but still large image at the normal reading distance of (usually)

16 inches. In the stereoscope we use the lenses to show us a “life

size” image at (a maximum of) an infinite distance, and other

similarly life size images at their correct, relative positions in

space. This orienting of objects in space does not conform, sub-

jectively, to magnification as usually understood.

However, lenses bend rays of light according to known laws.

Simple Ienses do not exert a uniform influence upon the rays, and

if the axis of the lens does not coincide with the original axis of

the image, the rays are not bent to the correct position. In short,

for results which are fully satisfactory, an achromatic lens should

be used and the instrument should provide such mounting that

the optical axes are maintained in their correct positions relative

to one another and relative to the optical axes of the stereoscope.

The Brewster Stereoscope.-In the Brewster instrument we

have another set of conditions. To understand the origin of the

lenses used, imagine a large lens whose diameter is twice that of

the lenses to be used in the stereoscope (Figs. 3-7 and 3-8). From

this large lens two smaller lenses are cut, one from each half as

A B

/‘ I\

c7 ., C /

I-. \

/ I

u ., C /

D E

Fig. 3-8. A, B and C show how lens is divided to cut out two smaller

lenses which are transposed and shown at D in edge view; and at E

in plane view.

shown in the illustration. These two derived lenses each have the
optical center of the original lens at their extreme periphery.
These two lenses are then mounted with their positions reversed,
that is with their thin edges adjacent. Thus for ideal results the