220 THREE-DIMENSIONAL PHOTOGRAPHYParastereoscopy is the nearest approach to orthostereoscopic re-
sults we can obtain combined with any effective telescopic effect.
It results from increasing the focal length of the taking lenses and
increasing the base in exact proportion to the focal length. A two-
times “para” involves a base of 130mm and lenses of twice normal
taking length. Any departure from this proportionate increase
throws the result into the general heterostereoscopic class.
Any true stereogram may be classed as one or the other of the
foregoing, and many of course meet the characteristics of two
classes, but at least it is a working classification which eliminates
much confusion and which has proven sufficiently definite for
practical use.