TRICK WORK 229is considerably more definite than in a pseudogram, although the
image is upside down.
A better, if more elaborate, method is to make a twin barrel
telescope and mount in the tubes a pair of Dove prisms. A Dove
prism is similar to a simple right angled prism with the apex cut
off. In fact an ordinary go degree prism may be used but the
unused apex takes up more room than the truncated Dove form.
The bases of the prisms are parallel, with the apices facing each
other. Light rays strike the first oblique surface, are bent toward
the base, reflected there, fall upon the second oblique surface and
thence to the eye. The reflection at the internal base of the prism
has produced a mirror type inversion, that is changed right to
left and vice versa. (This is also an excellent way to make a trans-
posing viewer.)
In short, the only thing required to produce pseudoscopic
vision is to exchange the right and left elements of visible sur-
faces! In the pseudogram, the right-eye image is seen by the left
eye and vice versa, but in direct vision the reversal is simply that
of the scene itself, as though reflected in a mirror. NO! You can-
not do this by simply looking into a mirror, each .eye must have
its own independent reflection.
The action of the Dove prisms is shown here:
The tubes of your pseudoscopic binocular need not be round.
In fact for experimental purposes you can make square cardboard
tubes. Or you can make cork mounts to slip into short lengths
of mailing tube. You can cut two holes in each of two pieces of
cardboard to hold the tubes parallel, and there you are. If you
wish a more permanent instrument you will find plastic as good
as metal and a lot easier to work with.
If you just want one look, you can lay the prisms on a table,
then kneel down and look through them. Be sure the shorter of