872 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



various errors in judgment of visual sensations frequently occur. Some 

 of these admit of explanation, others do not. The most striking example 

 of such an error in perception, and one which is the most readily 

 explained, is as follows : — ■ 



If a series of parallel vertical black lines, two millimeters in diameter, 

 are drawn on white paper, with equal white areas between them, and then 

 intently regarded in a good light, in a short time the lines will assume 

 the shape seen in Fig. 392 at A. They appear of this, shape because of 



Fig. 392.— Diagram illustrating Bekg- 

 mann's Experiment. (Stirling.) 



Fig. 393.— Zollner's Lines. (Stirling.) 



the manner in which the images of the lines fall on the cones in the 

 yellow spot, as shown in B. 



If a series of oblique lines are drawn perfectly parallel to each other, 

 and then they are crossed in different directions by a number of short 

 parallel oblique lines, although the long oblique lines are perfectly 

 parallel, the short oblique lines cause them to appear to slope inward or 

 outward, according to the direction of the oblique lines (Fig. 393). 



In the figure 8 and the capital letter S the upper half to most per- 

 sons appears of about the same size as the lower half. If, however, the 

 page on which they occur be inverted it will be seen that the lower part 



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Fig. 394.— Diagram illustrating an Imperfection of Visual Judgment. 



(Stirling.) 



is considerably the larger (Fig. 394) ; or if the centre of Fig. 395 be 

 fixed at about three or four centimeters from the eye, by indirect vision 

 the broad black and white peripheral areas will appear as small and the 

 lines bounding them as straight as the smaller central areas. 



If a disk similar to that seen in Fig. 396 is rotated the disk appears 

 to be covered with circles, which, arising in the centre, gradually become 

 larger and disappear at the periphery. If, after looking at such a revolv- 

 ing disk for some moments, it be attempted to read a printed page, or to 

 look at a person's face, the letters appear to move toward the centre, 

 while the person's face appears to become smaller and recede. If the 

 disk be rotated in the opposite direction the opposite results are obtained. 



