TYPES OF STEREOSCOPY 219HYPERSTEREOSCOPY.-This is a form of stereoscopy which is, as
pointed out by Helmholtz, to all practical intents and purposes
telestereoscopy. It is the result of using a stereo base which is
greater than the normal separation, sometimes as much as several
feet. In any event, the increase in base is disproportionately large
compared with the increase in the focal length of the lens (if any).
It usually denotes a wide base with the normal lenses.
HYPosTEREoscoPY.-This is just the opposite of hyperstereo and
denotes a base less than normal. Some stereographers erroneously
use a narrow base when making any stereogram nearer than ten
feet. The technique is of value when making very close-up photo-
graphs, at distances of 20 inches or less, when otherwise there
would not be sufficient overlapping field to produce a good stereo-
gram. Hypostereoscopic bases as little as a tenth millimeter have
been used in microscopic work, but for that purpose the method is
less satisfactory than the convergence or rotary method.
METAsTEREosCoPY.-This is commonly referred to as orthostere-
oscopy, but it must be remembered that orthostereoscopy not only
presents objects in full life size and at full natural distance, but
it does this for every object within the field. Metastereoscopy re-
produces one original object in its natural proportions, usually
much larger than life size; but when more than one plane is in-
troduced, objects lying in planes other than the principal plane
are not reproduced in the same proportionate relationships. It
might be said that metastereoscopy is the orthostereoscopy of a
single plane. It is typified by the carefully produced stereomicro-
graphs usually referred to as “ortho.” It might also be called the
normal stereoscopy of the microscopic field.
PARAsTEREosCoPY.-This is the peculiar composite of com-
pressed space and normal relief characteristic of the stereo-tele-
scopic field. Although the field appears to have only a portion of
its true depth, objects within that field appear to have their full
natural relief. Thus a three-times parastereoscopic field would
appear in such a manner that a wall 300 feet distant appears to be
only 100 feet away. That is, the space is compressed to one-third its
true value. But at the same time every object within that space
appears to have its full degree of relief, and does not appear to be
compressed to one-third its depth. Here we have a distinct separa-
tion of spatial relief and solid relief.