58 THREE-DIMENSIONAL PHOTOGRAPHY
of left lines mingled with patches of right lines, and the patches
will continually shift.
The old color anaglyph was based upon the assumption that a
red perception in one eye and a simultaneous green perception in
the other would produce white by the addition of color. As is well
known this method of stereoscopy proved a failure because of the
visually painful "bombardment." This effect is nothing more than
retinal rivalry which transmits first the green, then the red. Many
people however see only the color which results from the additive
mixing of the two colors, i.e., red and green (of correct hues) pro-
duce white.
Stereo-Cryptograms.-The stereo-cryptogram is another source
of amusement. In this, a series of letters are shown in solid block
Fig. 3-17. Cryptogram Christmas card. The stereo cryptogram adapted
to a Christmas card. Courtesy Robert Hayes.form in each unit (Fig. 3-17). Examined in the stereoscope certain
letters stand out before the mass or lie behind it, and so form a
message. Another form consists of an innocent message, certain
words or syllables of which are stereoscopically displaced to pro-
duce a hidden message.
Such cryptograms are made through parallax control. The sig-
nificant letters or words are written with different homologous
separation from the rest of the message. When the messages are
typewritten, the first message, the left for example, is written
straightforwardly. Then in the correct position, the second mes-
sage is written with the significant letters or words omitted. When
this draft is complete, the paper is shifted carefully in the carriage.