314 THREE-DIMENSIONAL PHOTOGRAPHYvalue of 32.5mm, which is equal to 3.25cm. As one meter con-
tains 100 centimeters, the deviation normal for a distance of one
meter is 3.25 prism diopters, or a total convergence of 6.5 prism
diopters.
There is no fixed angular value for the prism diopter. One
prism diopter has an angular value of about oo34‘23’‘, so we
should expect 100 prism diopters to have 100 times this value or
57” 18’20’’. However, by definition the distance through which
the beam is deviated is compared with the distance of the sur-
face upon which the deviation is measured, so at a distance of 100
units we should find a lateral deviation of 100 units giving us
two equal legs of a right angled triangle, so the angular deviation
must be 45 degrees for 100 prism diopters. The reason is that
each additional degree intersects the measuring plane at a greater
distance than the preceding one, so that each succeeding prism
diopter has a smaller angular value than its predecessor.
This method of angular measurement is considerably more
convenient in our work than the conventional angular measure-
ment.PRISM BASE
Another term often encountered in reference to stereoscopic
vision is “base-in” and “base-out,” with certain prism diopter
values. Thus we might encounter “base-in 13 diopters.” This is
actually the normal convergence for looking at an object at a
distance of 50cm. That is, if two parallel rays are deviated toward
each other by the use of a pair of prisms, each of 6.5 power, the
two beams would be deviated toward each other 6.5 prism diopters
and would have a total convergence of 13 prism diopters. If these
parallel rays are separated by 65mm, this 13 diopter convergence
will cause the rays to meet 50cm in front of the prism.
On the contrary, if you have a standard therapeutic stereoscope
with 200mm lenses and gomm base, one prism diopter will have
a lateral value of 2mm. If you prepare a stereogram for this in-
strument in which the maximum homologous separation is 96mm,
you have the condition of 3 prism diopters, base-out.
Thus “base-in” indicates convergence while “base-out” repre-
sents divergence, both related to the parallel vision of the normal
condition of rest.