432 



COMPARATIVE ELECTRO-PHYSIOLOGY 



instead of photographs, incised plates with two inclined cuts, 

 the right eye seeing the slit inclined to the right, while the 

 left sees that inclined to the left. When the observer looks 

 through the stereoscope turned to the bright sky, his two 

 eyes are acted on by strong stimulus of light through these 

 divergently-inclined slits. .The resultant sensation forms the 

 image of an inclined cross (fig. 262). On now closing the 

 eyes the multiple after-effect is observed as recurring images. 

 The relative difference of half a period, already referred to, is 

 here perceived in a very interesting manner, for the after- 

 image is not now the complete inclined cross. Instead of this, 

 each of the two arms is seen in regular sequence alternately. 



FIG. 262. Inclined Slits 

 for Stereoscope and Com- 

 posite Image formed in 

 the Two Eyes 



FIG. 263. Composite Indecipherable 

 Word, of which Components are 

 Seen Clearly on Shutting the Eyes 



That is to say, when the one is at its brightest the other has 

 completely disappeared, and vice versa. The impression in 

 each eye thus undergoes a periodic fluctuation. Some very 

 curious effects can thus be exhibited, when, instead of the 

 two inclined bars, the incisions to be looked through com- 

 pose a word. For instance, two letters forming half of the 

 word may be seen by one eye, and the other two by the 

 other. The result of this is that, a,s long as the eyes are 

 open, we obtain an indecipherable image, due to super- 

 position, like the following (fig. 263). 



But when the eyes are closed, then, owing to binocular 

 alternation of the multiple after-effect, the blur disappears, 

 and the word is spelt out in repeated succession in the field 



