Bauhaus-Universität Weimar

Experimental Psychology: A Manual of Laboratory Practice, Vol. I: Qualitative Experiments, part 2: Instructor's Manual
Titchener, Edward B.
§ 47- Stereoscopic Slides 
a similar phenomenon. Slide xxxi. is extremely baffling. One 
has a very distinct suggestion of tridimensionality, and the eyes 
soon grow fatigued in the attempt to ‘ set themselves ’ for a true 
binocular combination. The combined figure becomes flatter 
and flatter under steady fixation. The rivalry of contours may 
be noticed at the points of crossing of the boundary lines. 
Slide xxxi. — Half size. 
The phenomena to which this set of slides introduces us are 
those of the prevalence of contours, the rivalry of contours and 
lustre. (1) Slide xxv. gives a pure instance of the prevalence of 
contours. The white of the right-hand field is ‘suppressed’ by 
the A of the left-hand field. This fact may be generalised as 
follows : “ Any contour in the one retinal image assists the 
adjacent portions of the field to a permanent victory over the 
differently tinted ground of the other retinal image” (Hering). 
(2) Slides xxvi.-xxxi. show the rivalry of contours. “If contours 
in the two retinal images take such directions that there is 
retinal congruence for only a single point of each, they appear 
to cross one another at the point of the visual field which cor¬ 
responds to this pair of congruent points, but always in such a 
manner that the one contour (together with the strip of ground 
adjacent to it) interrupts the other or is interrupted by it ” 
(Hering). Hering finds the significance of the prevalence and 
rivalry of contours in the fact that “they prevent the fusion of 
the two retinal images, and secure to each a certain amount of 
independence.” Without them, the images of the two unmoved 
eyes would necessarily run together, as objects directly seen 
through an unmoved glass and objects mirrored in it run to¬ 
gether. They are, indeed, a sine qua non of the binocular per-


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