188 THREE-DIMENSIONAL PHOTOGRAPHYfarther apart. The theory is that if the farthest object in the films
is spaced as infinity is spaced in a full depth scene, then the nearest
portion of the object will not require such sharp convergence.
That theory is perfectly logical, but the first thing that comes to
mind is the fact that the degree of compensation permissible is
determined, not by the main object, but by the farthest object in
the picture. I$ this were not done, the images of the farthest object
might well be so widely separated as to make divergent vision
necessary. This is to be avoided because divergent vision is un-
comfortable and except in slight degree becomes painful, then
renders fusion impossible.
Compensation of this kind, then, is really limited to those
closeup subjects which have a very near background, usually one
which is not more than twice the distance of the main object.
What happens when the slide is projected? The parallax re-
mains the same, therefore the visual distance remains the same. At
the same time the demand upon the focus-accommodation bond is
not as great, This is a physical relief to those with bonded vision,
but it also leads to a vague dissatisfaction with the reproduced
image. It should by rights be in two positions at once, but this is a
minor defect and one which is not experienced by all spectators.
On the whole, compensation works through subservience to in-
ferior visual skill, and for projection before audiences who are not
familiar with stereo it undoubtedly has value. However the stere-
ographer who does his work for the benefit of himself and his
friends will probably prefer to stick to the straight para form.
It must not be thought that this compensation involves a differ-
ent basic type of projection. It can be applied to paraprojection
with the results described; it can also be applied to any of the het-
erogeneous forms of projection in which case it will not take from
such form any of its other objections.
Compensation is simply a compromise which is not particularly
harmful and which can be used at times to advantage, but this ad-
vantage is greatest when it is used as a modification of paraprojec-
tion.
STEREO PROJECTORS
There are two stereo projectors available. The Stereo Vivid has
been on the market for some time and has won general approval