Photoreception 



403 



land and are seen through the window. A variety of devices in deep-sea 

 fishes tend to increase visibility at low intensities. Some of these are relatively 

 larger eyes, relatively larger lenses and pupils, a greater concentration of rods, 

 tubular eyes which sacrifice peripheral vision for a more effective central 

 vision, and various patterns of bioluminescence to illuminate the objects to 

 be seen. 



Aerial Vision. The dioptric differences between eyes adapted to vision in 

 air and in water are pronounced and are discussed above. Another group of 

 differences include those pertaining to protection of the cornea, especially 

 from drying. In man and many other animals this is accomplished by blink- 



Fig. 113. Diagram showing the upward visual field of a fish, a, The aerial window as 

 seen from below; h, explanation of the aerial window, assuming calm water. Light rays 

 striking the water surface within the window are refracted, while those striking outside 

 the window are reflected. From Walls.^*' 



ing. In birds and many mammals, however, there is an additional structure, 

 the nictitating membrane, which probably covers the cornea most of the 

 time during flight and prevents drying by the rapid air stream to which 

 the eye is exposed. 



Air- and Water-Vision. Animals which live in both air and water must 

 have an exceptional range of accommodation in order to see v\'ell in both 

 media. A fish, when placed in air, is automatically subject to myopia, and 

 any air-living vertebrate, when under water, to hypermetropia. Aimhleps, the 

 so-called "four-eyed fish," has two pupils for each eye, and two retinas, one 



