THE STEREOSCOPE 57landscapes, owing to the confused overlapping contours, but a
rounded, solid object makes an attractive slide. (See Pictorial
Stereography.)
Iridescent slides are ordinarily hand drawn, one of black lines
upon white and the other of white lines upon black. The binocu-
lar mingling of the black-and-white produces a scintillating, irri-
descent effect.
Similarly, a pseudo-stereogram of some subjects such as a foun-
tain or a waterfall, one image made in dull light, the other in
direct sunlight, will have no real stereo relief. However, due to
retinal rivalry the sparkling image will conflict with the dull one
and often the effect is that of moving water.
Retinal Rivalry.-It is impossible for two widely different im-
ages, transmitted by the two retinas independently, to be simul-
taneously perceived. This fact should not be confused with stereo
Fig. 3-16. Retinal rivalry. The eye does not see a true crosshatch, but
alternating vision in small patches of first one set of lines, then the
other.diplopia, for there the dual images are seen by oblique vision. But
when two different objects are the subject of direct macular
vision, first one image will be seen, then the other. The successive
phases may last for several seconds, or for only a fraction of a
second. This is best illustrated by a dual slide in which there are
oblique lines at right angles in the two units (Fig. 3-16). Instead
of a perfect crosshatch pattern, the area will show small patches