APPENDIX ATHE PRISM DIOPTERIn stereo, we are vitally concerned with two visual axes which
converge. Parallel visual axes do not give any stereo parallax,
hence are useless in stereo. That is, objects at infinity lie beyond
stereo infinity. For that reason we are concerned only with con-
vergent axes and with light rays which have other corresponding
angular relations.
It is commonplace to use the prism diopter as the unhgf meas-
urement, rather than the conventional degrees, minutes and sec-
onds.
The prism diopter is a prismatic power which will cause a
beam of light to deviate to the extent of one part in one hundred.
Thus if you project a target upon a screen at a distance of loo
inches from the projector, and mark the position of the center
of the target; and if you then place a prism over the lens of the
projector which causes the center of the target to fall one inch
at one side of the original position, you have a prismatic devia-
tion of one diopter, and that prism is said to have a power of one
prism diopter.
Of course, if the prism causes a deviation of one inch in 100, it
will produce a deviation of one centimeter at a distance of 100
centimeters or a deviation of one foot at a distance of 100 feet,
It is customary to give the deviation as applied to one ray only,
that is, if each eye deviates one prism diopter, we have actually a
total convergence of two prism diopters, but we speak of the con-
dition as a deviation of one prism diopter. Note that “deviation”
is the movement of one eye, “convergence” is double the deviation
and applies to both eyes. This is not a fixed rule and often in
optical literature you will find the terms used interchangeably,
with considerable resulting confusion.
THE HALF ANGLE
This is similar and refers to the monocular deviation. The base
is the distance from the pupil to the center of the nose, or one-
half the interpupillary distance. Conventionally the base has a
313