MOUNTING, TRANSPOSITION AND SPACING 79tinued, cross at a distance equal to the true distance of the original
object.
Thus the stereogram is not a physical object to be visually ex-
amined. Rather it is simply a guide or stencil which when visually
projected causes the two separate images to fuse at some point
out in space equal to the original distance of the object. This
visual projection, as can be seen from the diagram, is a very real
thing and not at all fanciful theory.
As we have seen, direct stereoscopic vision is possible only by
virtue of a psychological visual projection, so this geometric visual
projection is quite simple, yet it is the keystone of all stereoscopy.
Thus it will be seen that the photographic image must be in-
verted to cause it to correspond to the original picture plane im-
age. However, in actual practice it is not usual to revolve the
images after they are cut apart.
METHODS OF TRANSPOSITION.-GGUss-bUsecl images. When glass
plates are used in the camera, and when the positives are printed
upon glass, the transposing printing frame is used. This is a frame
which is longer than the negative, sometimes half as long again,
sometimes twice as long. The frame which is as long as three
single images is the most common. The negative is placed at the
left side and the positive at the right. The two overlap in the cen-
ter where the printing opening of the frame is located. The
exposure is made. The frame is opened and the negative is pushed
to the right while the positive is pushed to the left, and again
the exposure is made. Thus the left negative image is printed at
the right of the positive and vice versa,
It can be seen that this principle presupposes the use of rigid
materials which will conform to the limits of the frame used, but
glass is not the only material that can be used. If the positives
are made upon stiff sheet film, the transposing frame may be
used, and even when both negative and positive are of roll film
it may still be used if care is observed.
Film negatives. It is customary when the negatives are made
upon film, to cut the two images apart, reverse the positions and
re-attach the two images with a strip of adhesive tape. Quarter-
inch cellulose tape is excellent for the purpose, although if the
negatives are not kept in individual envelopes, they will stick
together in time,