CHAPTER 18HYPERSTEREOYPERSTEREO WAS BRIEFLY MENTIONED in an earlier chapter,
H but because of its importance and because the subject has
been the cause of so much bitter argument, it will now be dis-
cussed in detail.
It will be remembered from the preceding discussion, that par-
allax is the determinative factor in the appearance of distance.
The greater the parallax, the closer the subject appears to be. Al-
though not in accord with the classic theory of stereophotography,
parallax is the sole determinative factor, any telescopic effect re-
sulting from the use of long-focus lenses being insignificant. This
fundamental fact was recognized years ago by Helmholtz who
constructed an instrument closely similar to the stereo reflector,
but made for direct visual use. This instrument he called the
telestereoscope. It gives the absolute distance effect indicated by
the proportion of its base to the normal base. Thus if such a re-
flector is made with a 65cm base, objects will appear to be ten
times nearer than when it is not used. However, when used for
direct vision, we have no telescopic means to enlarge the per-
spective value of the scene, therefore it is obvious that the objects
viewed will appear to be small, only one-tenth natural size in our
example. The whole thing will give the effect of looking at an
accurate miniature model very near at hand.
This fact has led, together with sound stereo theory, to the state-
ment that the use of hyperstereo produces a stereogram which has
the appearance of being a miniature model, a short distance away.
That argument is based upon a fallacy which is not obvious
until it is closely studied in the light of what little we know about
subjective react ions.
In the instance of the direct vision telestereoscope, we have the
direct comparison with the natural aspect made possible by look-
ing through the instrument, then looking at the same scene with-
out it. The comparison is made with the minimum time interval,
and for this reason, the appearance described can be observed.
The stereogram offers no such comparison with the natural
aspect, and for that reason we can only guess at the normal appear-
260