246 THREE-DIMENSIONAL PHOTOGRAPHY
of the retinal surface, but aside from such complex and costly
instruments, the work requires two cameras or a single camera
used to make successive exposures.
The two cameras should naturally have sufficient bellows capac-
ity to make possible images at least 1: 1 in size, and it is preferable
if this can be increased. The ideal equipment is bulky, because
it consists of two cameras of great bellows capacity equipped with
lenses of long focal length, Eight-inch lenses on cameras of some
30-inch bellows capacity are excellent, but the lens boards should
be small, if a diagonal is to be used.
The DiagonaZ.-This is some kind of beam splitting device. It
may be a normal beam splitting cube, but unless it has great size,
the camera lenses must be in extremely close proximity, and there
is only a minimum base available. For this reason, the beam
splitter is usually restricted to special instruments used for one
type of subject only.
A better device is a 50-50 mirror. This mirror transmits 50
percent of the light and reflects 50 percent (actually there is a
slight loss in transmission, but due to the coating used this may
be ignored). The better mirrors ars dichroic, however, and in
color work this is somewhat objectionable as there is a pink cor-
rection applied to one and a blue to the other. It is said that this
slight color difference is merged visually by the observer, but the
writer has found a discernible effect.
However, the mirror does permit the base to be altered at will
and a iGx3 inch mirror will permit from zero to almost two
inches base.
In using the diagonal, the cameras are positioned at right
angles. One of the cameras accepts the image through the mirror
(nominally a direct image), and the other accepts the image re-
flected from the surface of the mirror at right angles. Displacing
either camera laterally will alter the base, but the motion is
usually given to the camera which lies normal to the object, and
that is the camera which receives the transmitted image. The mir-
ror is placed between the lenses at^45 degrees to both of them.
Double Diagonal.-This is the method favored by most workers,
although it has the disadvantage that zero base cannot even be
approached. Each camera has a right angle prism attached to the
lens, and the cameras point directly at each other. Thus BOTH