870 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



are not precisely the same. If we hold up any object and look at it first 

 with the right eye, having the left eye closed, and then with the left eye, 

 having the right eye closed, it will be readily seen that the picture is not 

 precisely the same. With the right eye we see the right side of the 

 object, and with the left eye the left side. When these two pictures 

 come to be fused in the brain there results an image which differs from 

 either of the other two and which is projected into space, giving the 

 idea of one solid body (Fig. 388). 



It is upon this principle that the stereoscope is constructed. The 

 stereoscopic picture is composed of two images, one representing the 

 object as seen by the right eye, and another representing the object 



as seen by the left. When these two 

 pictures are fused the resulting picture 

 is formed, which is a compound of the 

 two, and this fusion of the two images 

 gives the idea of solidity of the object. 

 When the e}^e has looked at an ob- 

 ject for a long time, especially if that 

 object be luminous, the retina becomes 

 fatigued and no longer capable of re- 

 ceiving impressions from it. If the 

 object be small only a small portion of 

 the retina will be impressed. If we 

 turn away from the object and fix the 

 eye upon a white wall we see a dark 

 spot upon the wall corresponding in 

 size with the object upon which we 

 have been looking. Suppose that we 

 fix the eye upon a bright red wafer 

 FIG ' ^u^R G vfswN LL 7Sa G Bl " strong^ illuminated, and look at it 



The lines from the object indicate that the rays Steadily With OU1* eVCS, tllC raVS DrO- 

 from the back of the book fall on coincident points of 



of^rilfon*' while each eye> further> has a special field ceeding from that wafer and falling upon 



one point of the retina will fatigue that 



point so that it will not be capable of receiving rays from less luminous 

 objects, and when we turn our eye to the wall we see a spot on the wall 

 like the wafer in size but of a different color, made up of all the colors 

 of the spectrum except red. This combination constitutes what are 

 termed accidental or complementary colors. Again, if we look at 

 a white object for a long time and then look at the wall we see there an 

 object of the size of the original one of a black color. The point of the 

 retina upon which the image fell has become so fatigued that it cannot 

 receive an impression from the fainter rays reflected from the wall, while 

 all the other parts of the retina are impressed by these rays. We, there- 



