THE EYE AND THE PHYSIOLOGY OF VISION. 



583 



the retina. A somewhat curious exception to this rule of 

 double vision is found in viewing the starry heavens. Here 

 all the stars appear as single stars, whether the image falls 

 close to the yellow spot or on the periphery. This arises 

 from the fact that owing to the immense distances of these 

 objects the rays coming from them are practically parallel, 

 and entering the eye in parallel lines all the images must 

 naturally fall upon corresponding points. Reference to 

 Figure 183, in which the rays of light from one star are 

 given as lines 1 and 7, and rays from the second star given 

 as lines 2 and 2, it will be seen that c and c and a and a 

 are corresponding points. 



c * c, 



Fig. 183 DIAGRAM SHOWING THE COMPLETE CORRESPONDENCE OF RETINAL POINTS IN 



LOOKING AT VERY DISTANT OBJECTS, SUCH AS STARS. 



THE ADVANTAGES OF TWO EYES. 



Even if one eye were able to see all colors as perfectly 

 as two, one or two decided advantages arise from the pos- 

 session of two eyes. 



First. The defect in the retina caused by the blind spot 

 is remedied by the opposite eye, as the corresponding points 



