STEREO PROJECTION 193b
Fig. 12-6. Box mask for crossed axes viewing; b shows the shape of
the box when two front apertures are substituted for a single one,
making the box much shorter.
Although these primitive devices have actually been used, it has
been found that the substitution of optical elements for the open
visual paths improves the instruments. Fig. 12-7 is the simplest,
the insertion of a prism to deviate one beam. However because
this introduces a non-linear error it is not a truly practical form.
12-8 shows two prisms, which prove better because the error is
split between the two. The smaller figures b, c and d show how
rotating prisms provide for control of the degree of divergence.
Fig. 12-9 shows a pair of dove prisms used for this purpose, a
method which is better than the preceding, but which involves
the use of non-transposed pictures because dove prisms form a
transposing optical system.
The form which was actually made for commercial distribution
and used for viewing projected X-ray pairs is that shown in Fig.
12-10. Here two pairs of prisms, the outer members of which are
adjustable as to position makes it possible to superimpose even
rather large pictures without visual optical distortion. Unfor-
_....- .....a*
Fig. 12-7. Convergent vision with single prism.