THE STEREOSCOPE 41I-H separation of the stereogram should be equal to the distance
between the optical centers, or the extreme outer edges of the
two small lenses.
When adjusted for normal vision, the stereogram is supported
in a plane whose distance from the lens is equal to the focal
length of the original lens. Now we shall see why this viewer re-
quires no interpupillary adjustment.
If the instrument is set up and a plain card placed in the sup-
port and a parallel beam of light is then projected into the lenses,
they will bring the beam to two foci which are separated by a dis-
tance equal to the distance between the optical centers of the
lenses and equal to the normal I-H distance. All parallel rays
falling upon any part of the lens are brought to the focus. This
is a characteristic example of the use of a collimating lens, or
using a lens to produce a parallel beam from a point source, or
vice versa.
As the human eye at rest is in a condition of zero convergence
and accommodation, the visual axes are parallel. Thus no matter
what the interpupillary, these parallel visual axes will coincide
with a pair of rays in the parallel beams, and the visual axes
will be projected to the optic foci by the collimating action al-
ready described. Thus, regardless of interpupillary, all eyes will
react in a similar manner.
The choice between the full lenticular and the Brewster (or
semilenticular) instruments has led to better argument, both
schools having adherents, but a little thought will show that they
are optically very similar, and that when the interpupillary is less
than normal, the viewing conditions of both types are identical.
The real choice lies in stereogram size, or rather homologous
separation. If the separation is greater than the normal 65mm,
the Brewster is used. The whole point is that the optical centers
of the lenses should be separated by the distance equal to the
homologous separation of the images. The Brewster type would
work just the same if full lenses were substituted for the half
lenses, but as the external halves could never be used, why
waste glass?
Thus, any full lenticular viewer may be used as a Brewster by
separating lenses and stereogram images to any distance greater
than the normal interpupillary of the observer. Likewise, in a