Instead of cutting the positive transparency and then transposing the images, it is better to print in two stages (Fig. 205), printing on the right half of the plate the right-hand image which appears on the left-hand side of the negative viewed in the normal way, and then on the left half of the plate the left-hand image which appears on the right side of the negative. The name " reN, ersing frame " by which the printing frame used in this transposition is known, and the instructions which accompany its description in some catalogues, have led some photographers to imagine that for correct vision of a stereoscopic pair one must present the image formed by the right-hand lens to the left eye, and vice versa, whereas the transposition is made solely for the purpose of placing the left image before the left eye. Without this precaution, the left image would appear in front of the right eye, and vice versa.
817. Pseudoscopic Pairs. When two perspec tives of a geometric solid are viewed in a stereo scope, the left perspective being presented to the right eye, and vice versa (e.g. the two draw ings of a dodecahedron reproduced in Fig. 206), the faces which in normal vision appear in front will appear behind (and vice versa), the reconstructed solid being thus the inverse of the original object, the reliefs appearing as hollows and the hollows as reliefs. This phenomenon is generally known as pseudoscopy.
If a pair of stereo pliotographs whose images have not been transposed is examined in the usual way (e.g. if the pair consists of the two
images produced in a stereoscopic camera fitted with two lenses), an analogous phenomenon tends to occur, but very often the stereoscopic effect is opposed to the perspective, and some When one of the negatives of a stereoscopic pair is useless, an amateur will sometimes take two prints from the good negative, making what is known as a planoscopic pair. The viewing of such a pair in the usual stereoscopic manner, each eye occupying the viewpoint of one of the images, which is merely the normal condition of monocular vision of a photograph 27), often gives a certain appearance of depth ; it is not comparable with the true stereoscopic effect, but sufficient to cause a planoscopic pair to pass times even contradicts common sense, an object partially masking another often appearing behind the object it hides.
There are, however, cases where the inversion of relief by the examination of pseudoscopic pairs affords a useful control of deductions made from stereoscopic photographs' by all the effects of auto-suggestion in the percep tion of shape. Such is the case, for example, in radiography, where pseudoscopic vision may be used to reverse the perspectives, thus bringing the further objects to the front for examination.
Planoscopic Pairs. Two identical pers pectives 2 obviously cannot give a stereoscopic effect, for there is no variation in the convergence of the optical axes when objects at different distances are examined.