Three-Dimensional Photography - Principles of Stereoscopy

THE STEREOSCOPE 39

will provide far more satisfaction than a $300 camera and a $5
viewer! Practically every disappointed beginner can trace his
disappointment to the use of a cheap or faulty viewer.

stereo viewer is not difficult to understand, but once understood,
the necessity for the best possible instrument will be appreciated.
Lenticular Viewers.-The normal viewer, that is, the viewer
used with stereograms whose I-H separation (homologous sepa-
ration of objects at infinity) is 65mm, is usually called a lenticular
viewer. The lenses are used as simple magnifiers, but not for the
purpose of making the image appear larger.
The lenses serve a dual purpose. First they magnify the images
to just that degree needed to compensate the diminution or nega-
tive magnification of the short focus camera lens, and second to
bring the image into sharp focus while leaving the eyes in the
condition of rest. (Infinity focus = zero accommodation.)
Thus while there exists the magnification incident to viewing
anything through a positive lens, this magnification, under or-
thostereo conditions has no significance because it only brings
back to normal dimensions an image which has been made small
in the camera.

OPTICAL CHARACTERISTICS OF VIEWERS.-Th€! function Of the

F

I

b

C

Fig. 3-7. A large lens focuses parallel rays a to g at F.

If lens is split and the halves transposed, the upper half

focuses rays d-g at Fl; lower half focuses rays a-d at $2.