CLOSE-UP STEREOGRAPHY 253(surface of the slide). A screw adjustment is provided for con-
trolled motion. This stage is clamped to the regular microscope
stage, and rotation obtained by tilting the stage between’ the
screw-set limits between exposures.
The writer uses a somewhat different setup for this work. The
microscope used is a petrographic instrument with high stage
clearance. Upon this is mounted a petrographic “universal stage,”
which permits the object to be moved in all directions. Thus the
final orientation-so difficult to obtain when mounting a speci-
men-is obtained and the position fixed by locking the stage rings
involved, and leaving the main axis free. The required tilt is then
obtained by using the main axis, which is calibrated in steps of ‘/1
degree. The only objection is that such a setup is far too costly for
any but the serious worker.
It might be added that the petrographic microscope is used
solely because it has working distance to accommodate the uni-
versal stage. Polarization is not normally employed, and apochro-
matic objectives from a biological instrument are used.
Many biological microscopes can be raised high enough to ac-
commodate the universal stage, provided objectives are used
whose focal length does not exceed 32mm.
The illumination is provided by special “light funnels” which
concentrate the beam upon the surface of the object. Most ob-
jects transparent enough to be used with transmitted illumination
have insufficient relief for stereo. It is obviously difficult to re-
produce a relief which does not originally exist.
Another factor must be considered when the object is
“mounted” by being sealed in glycerine, balsam or the like. These
substances have a greater refractive power than air, so the uppar-
ent relief is diminished, sometimes to the extent of 2/3 of the
true depth. When making stereos of mounted specimens the
rotation should be increased by at least 50 percent to compensate
for this loss, if image definition will permit.
The tilting stage is placed upon the microscope stage and fixed
to it. Using the centering adjustment of the main stage, and the
adjustment of the auxiliary stage together, center the tilting stage
so that the crosslines are centered in the microscope field, and
remain centered when the main stage is revolved.
Set the main stage so that the crosslines of the tilting stage are