CLOSE-UP STEREOGRAPHY 255the laboratories of the Stereo Guild, is based upon the use of a
variation of the Zeiss stereo diaphragm which is used beneath the
condenser. It might be added that it is important to use a good
achromatic condenser and to be as careful of condenser focus as
of objective focus. (Figs. 17-7, 17-8, 17-9.)
The Guild controls consist of a substage diaphragm in which a
go degree sector is open. This sector does not extend clear to the
center. In the full illumination (bright field) the solid central
disc is about 3mm diameter, but when used for dark field, the cen-
tral solid disc is of such size that the full field of the objective is
blocked. In short, this diaphragm is just like the usual dark-field
diaphragm, but instead of having almost full annular openings it
has only the go degree opening. Similar but small diaphragms
are made to be inserted just above the rear of the objective, inside
the tube. The position of the tube sector in relation to the con-
denser sector is reversed for changing from bright to dark-field.
Although some improvement seems to be apparent in some in-
stances, the value of such diaphragms is questionable owing to the
great difference in lighting effect in the two images, far greater
than is normal for binocular vision.
In general, in all macro and micro stereo, as in normal photog-
raphy, the photographic technique is not altered. The stereo ele-
ment lies in making two exposures of the object with parallactic
differentiation produced by alteration of physical relation to the
camera lens.
arises but one factor of importance; the computation of the degree
of change. In general, it may be said that the base changes in
proportion to the distance of the object from the lens, but any
such basis, although it is widely used, is at best vague. In normal
stereo we use a fixed base for distances between about five feet or
less and infinity. Thus the first question which arises is whether
you will use infinity or five feet as your reference base. That, in
itself, introduces too great a variation. Therefore we make use of
a definite reference base, and this is derived from the value of the
parallactic angle.
In any anortho stereo, the first question to be decided is, “At
what distance shall the object appear?” You will keep in mind that
the true distance factor in all stereo is the parallactic value. There
DEGREE OF DIFFERENTIATION.-In all Of these methods there