252 THREE-DIMENSIONAL PHOTOGRAPHYalready used to designate a different appearance. There is a
definite relief. The relief is associated with true relief, but does
not accurately reproduce it. False relief is sometimes introduced.
Most important of all, the effect obtained by lighting control
falsifies the true appearance because in stereo, particularly in
stereo close-up, there is a difference in the lighting caused by the
different angles of reflection to the two eyes, an effect which we
call illumination parallax. Just as false stereo parallax will exhibit
some kind of relief (spurious), so will false illumination parallax
produce spurious relief.
Therefore, it is advisable to use illumination control with re-
straint and only as an adjunct to physical displacement.
Lateral Displacement.-Due to the great magnification, care
must be used in making the displacement or the prints will have
to be carefully aligned later. The most important thing to remem-
ber is that the displacement is lateral and only lateral. If the
object is moved diagonally trouble will ensue.
Some kind of guide bar against which the slide may be moved
is practically essential. Fortunately most modern instruments are
equipped with a mechanical stage. If this is set to traverse the
line of displacement, all that is necessary is to move the stage. The
calibrated scales make it easy to move the object through just
the distance required.
The motion is controlled by examination of the focusing screen,
otherwise the displacement might move the object out of the
field. Also because in both exposures the object lies toward the
edge rather than the center of the field it is highly advisable to use
apochromatic objectives with special photographic (flat field) ocu-
lars. If this is not done, the curvature of field will introduce image
distortion which may be serious if some parts of the object
approach the field edge. For this reason, and because the micro-
scope field is very shallow, it is preferable that rotary displace-
ment be used.
Rotary Displacement.-Rotary displacement is not difficult with
the microscope. An ordinary slide may be rotated by propping
the end upon a match stick, with the opposite ends being raised
for the two exposures. However, this is too crude for good work.
Some English microscope makers (e.g. Watson) make stereo stages
in which the slide lies in a cradle pivoted in the optical plane