216 THREE-DIMENSIONAL PHOTOGRAPHYamateur has seen examples of such projection. Inasmuch as we
have already described this procedure in some detail in an earlier
chapter, we shall not repeat the description. It will be remem-
bered that the two stereo unit images are projected through two
polarizing films whose axes lie at right angles: that they are pro-
jected in approximate superimposition upon a metallic surfaced
screen; and they are viewed through spectacles composed of two
polarizing films whose axes are at right angles and which are
oriented with the projection axes to produce the necessary can-
cellation.
THE VECTOGRAPH.-A different type of polarized stereogram,
and one which is both deeply interesting and commercially prac-
tical, is the Vectograph. We are not going into the detailed theory
of the Vectograph, because it is somewhat involved for those un-
familiar with the simpler phases of polarization, and because the
stereographer has no call to make use of such knowledge.
Vectograph in Projection.-The Vectograph, as first made, con-
sists of the substantially transparent base with a different image
upon each side of this base. It can easily be understood that it can
therefore be projected, just as any transparency or lantern slide
is projected. We also know that polarization is not affected by
projection per se, so if we use a standard aluminum-surfaced pro-
jection screen (which does not depolarize the light), we have in
this stereogram a slide which may be projected normally and
viewed with the standard 3-D goggles. Cloth, beaded screens,
plaster, and the like will show both images even when the correct
viewer is used.
Inasmuch as machines were long ago developed for printing
motion-picture films in two colors for the early color processes,
there is no reason why Vectographic motion-picture films could
not be made, thus providing stereoscopic films which could be
projected with any standard motion-picture projector and viewed
with spectacles as easily worn as ordinary sun glasses.
Vectograph as u Transparency.-Because there is no reasonable
size limitation, large Vectographs can easily be made and dis-
played as transparencies in public places. These would be espe-
cially suited for demonstrations at industrial exhibitions, for
example, and in museums. Instead of individual viewers, a series
of strips of polarizing film about two and a half inches wide run-