CHAPTER 3THE STEREOSCOPEOST STEREOGRAMS MUST BE VIEWED with some device which
M causes the two eyes to see their respective images optically
superimposed. This is true without exception of orthostereograms.
There are some types of photographs which incorporate dual
images which are seen in relief without any viewing device,
but this does not make them orthostereograms, because although
they exhibit relief they do not fulfill the fundamental require-
ment of exhibiting the object in full natural size at full natural
distance.
Many stereographers have developed the ability to see a stereo-
gram without a viewer. This is the result of so training the eye
muscles that accommodation and convergence are separated, so
that the eyes can focus upon the near-by stereogram while retain-
ing the zero convergence of infinity viewing. This type of stereo
vision approximates the ideal more closely than the free-view
devices, but it is still not orthostereo.
The device used for viewing is generally called a stereoscope.
There is a widespread but inexcusable error of calling it a stere-
opticon, which means simply a lantern used for projecting lantern
slides; specifically, the double-dissolving type now almost extinct.
Do not fall into this error.
The stereoscope may be a simple, folding arrangement of card-
board carrying two simple magnifying lenses, or it may be a
beautifully finished piece of furniture containing an elaborate
mechanism for displaying the stereograms in any desired order.
Between the two extremes there are many different forms.
The first difference is based upon size, of which we have far
too many. The sizes are divided into two great groups, those
which are spaced to correspond to the average eye spacing of
65mm, and those which are spaced farther apart than the normal
interpupillary distance.
NORMAL SPACING.-NOrmal spacing starts with the 16mm film.
These are either printed upon a length of film with the units of
various pairs interlaced in sequence, or as in the Sawyer View-
master with seven pairs of pictures spaced about the periphery
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