SPECIAL PHASES 293journals which were based upon fallacy. As a result, there are a
few specialists who have done excellently well in stereo X-ray,
some who obtain satisfactory results and the great lnajority who
fail to obtain usable results.
The technique involved ordinarily makes use of two exposures
from two tube positions. These tube positions are based upon the
theory of convergence, but by the very nature of the X-ray emis-
sion they are fundamentally comparable to the parallel ray tech-
nique of the standard dual camera. This is because the emission
of the tube is in all directions within the scope of the mask used;
or, in other words, almost the exact reverse of the rays impinging
upon the camera lens. However, there are no truly axial rays, so
even with the actual tube converged the result is substantially that
of parallel axes.
The result, of course, is a highly variable relief depending upon
the tube distance versus spacing, It is a simple problem to one
who knows stereo theory thoroughly, but the fact that operators
believe the process to be extremely complex makes them commit
errors.
Recently, a new technique has been introduced which promises
to clear up a great deal of this confusion. This “new” technique is
nothing more than the adaptation to X-ray of the long and favor-
ably known rotary displacement, so common in stereo micro work.
The subject is placed upon a table which may be slightly ro-
tated, or the exposed hand, foot or head is simply rotated between
the exposures. This gives a full control of relief through angular
displacement, and the distance factor has little significant effect.
If the films are to be viewed in the normal Wheatstone viewer
with an eye-to-film distance of three feet, the angular value of the
rotation will be normal to human vision at three feet. Nor does
the rotation have to be exact. Twice the normal rotation will not
produce too much exaggeration. For locating foreign bodies, of
course, more precise control of angle is desirable.
Thus, while a great deal of space could be devoted to a discus-
sion of the older technique, it is suggested that those who are
interested in stereo X-ray work try this new technique, and for
experimental purposes try rotating the subject through approxi-
mately two degrees each side of center, a total excursion of four
degrees, which will place the image in space at a distance of ap-