CLOSE-UP STEREOGRAPHY 257Using the five-degree value, a tilt stage will be inclined to five
degrees each side of center for the two exposures, or a total of ten
degrees. If the specimen is “permanently mounted,” the swing will
be about 7.5 degrees each side of center.
For lateral displacement, the distance moved is one which will
correspond to the length of the short leg of a triangle of five de-
grees apical angle, whose long leg has a length equal to the
focal length of the microscope objective used. This is true be-
cause when a microscope is correctly focused, it is focused for
infinity and the lens distance is actually the focal length. (The
free working distance is less because the optical center lies within
the lens.) Thus with a 16mm objective, the long leg RS would
be a 16mm. Angle ORS is five degrees (or 7.5 for a mounted
object), and the motion through 0s is the desired distance.
Remember that 0s is the distance from center. 0’ is the second
position of the object and OS= 0’s.
LIMITATIONS.-There are always limitations to be considered
and in micro work these are somewhat severe. For example, when
a stage is tilted, the sides of the field are out of focus because one
side is too near the objective (inside focus) and the other is too
far from it (outside focus). Therefore, the amplitude of swing is
limited and the greater the magnification the less is the tolerance.
Even with the 16mm objective, one commonly used for stereo
micro, it has been found advisable to obtain a special mount
which incorporates a small iris.
It has been argued that this fault proves the superiority of the
lateral displacement, but the loss of definition and distortion at
the edges of the field, even with apochromatic objectives and flat
field oculars, is just about as bad as the violation of field depth
limits.
Secondly, high-power stereo, on the order of 1000 diameters, is
almost worthless because the field is too shallow to permit the
variations in depth which make up relief. The work then de-
scends to the level of making a stereogram of a printed page. If
the aperture of the immersion objective is lowered to a degree to
make possible any penetration worth while for stereo, the loss of
definition is too great. In our work, while we have made stereos
at relatively high power, we have found that a good stereo is
rarely obtained at powers above 200 diameters.