248 THREE-DIMENSIONAL PHOTOGRAPHYprism interferes with the orientation and lighting of the object.
Thus, for greater magnification it is necessary to work with a
single camera and successive exposures.
ROTARY DISPLACEMENT.-The argument is often made that be-
cause the human eyes converge upon an object, the stereo camera
should also be made to do the same. We have explained this
fallacy elsewhere, but when the object is limited to a group of
closely associated planes, the method is quite satisfactory.
In Fig. 17-6, the object lies at 0 and the two camera positions
are indicated as C and Cā. It is easy to see that what actually hap-
pens is that the camera is moved along the arc XY even though in
practice the camera is moved laterally and then turned to bear
upon the object. What happens is that the camera has two posi-
tions, the difference between which is equal to the parallax used.
When this is considered, it will be seen that, insofar as the ob-
ject is concerned similar results will be obtained by rotating the
object through the same arc. The object is often easier to move
than the camera, and this is a method widely used in higher power
macro and in micro stereo. It will be noticed, of course, that
when the camera is moved, the parallax involves all objects within
the field, while with object rotation only the object itself is
affected. Thus, with camera rotation the effect is closely similar
to that obtained by normal stereo displacement, but when the
object is rotated, only the object will have relief and the back-
ground and surroundings will be astereoscopic. Therefore, object
rotation is limited in most instances to a single object mounted
upon a neutral background.
MIcRo.-When the image is some ten times the size of the ob-
ject or more, we enter the field of photomicrography. This as-
sumes the use of a microscope in connection with the camera. In
fact, the fields of macro and micro overlap. It is possible to ob-
tain many magnifications with macro methods which can be
obtained with the microscope.
There was a time when photomicrography involved the use of
only the objective of the microscope, and to that extent it was no
more than an additional degree of macro. However, modern
photomicrography assumes the use of the complete compound mi-
croscope, although at times the visual ocular is replaced by a
special photographic lens system,