254 THREE-DIMENSIONAL PHOTOGRAPHY
accurately “up-and-down” and “right-and-left.” Focus upon the
crosslines with the objective to be used. Tilt the stage from side
to side. Note that the vertical line remains motionless and in
focus. If it seems to sway to one side or the other or to go in and
out of focus, it is not correctly oriented. Readjust until the tilting
does not affect the position of focus of the vertical line.
Observe the lateral crossline. Note that it rapidly loses defini-
tion as the stage is tipped. Tip the stage until the good definition
and diffusion are about evenly distributed along the line. Near the
center the crossline will be sharp, but halfway to the edge of the
field it will begin to soften, and at the edge of the field it is
definitely diffused. Note the angular displacement as shown upon
the scale. Now tip the stage in the opposite direction an equal
amount and see if the distribution of focus is the same. If it is not,
reset the stage to the correct diffusion and read the scale.
If, for example, the right tilt is six degrees and the left tilt is
five for equal lateral diffusion, the true “zero” would be the
mean of +6 and -5, or +0.5. Then if you wish to make use
of a six-degree convergence you add three degrees plus and
minus to the true zero to obtain the two stage settings, 3 + 0.5 =
3.5 and -3 + 0.5 = -2.5, so your settings would be 3.5 degrees
right and 2.5 degrees left. With this preliminary adjustment, you
are ready to proceed with the photography.
In macro work it is customary to use tilts of from four to six
or even seven degrees at each side of zero, but in micrography
with the microscope the amount of tilt is limited by the depth of
field, so that it will often be necessary to limit the tilt to as little
as two degrees each side of zero, or a total convergence of four
degrees.
The microscope field is always considered to be at the standard
reading distance corresponding to a total convergence of some
nine degrees, but if we are limited to a four-degree total we place
the object stereoscopically at the equivalent of some three feet or
a little more, which means a relatively flattened relief, while the
flattening effect of any mounting medium increases the flattening.
Thus there is a definite stereo limitation imposed by increasing
magnification. The greatest tilt allowed by the limits of diffusion
imposed by the lateral line diffusion should be used.
Illumination Control.-Illumination control, as developed in