376 ADAPTATIONS TO MEDIA AND SUBSTRATES 



away at his own level, it is a red-letter day for him. With increasing 

 depth, or increased turbidity, this distance is still further reduced since 

 the absolute amount of light reflected into the eye of the animal depends 

 upon the relative amount of sunlight reaching that depth. 



Under water, vision is handicapped while other senses are actually 

 promoted. It is not surprising that the fishes are better able to get along, 

 if blinded, than any other vertebrates. They use their eyes when they can, 

 but most fish can find enough to eat without seeing the food. Many fishes 

 are deprived of vision in winter, when the ice above them is blanketed 

 with snow, and some arctic species live out their whole existence under 



Fig. 128 — Fish from above, showing visual angles in the horizontal plane. 



This particular fish does not have complete periscopy — with a less bulky body the posterior 

 blind angle would diminish; but the anterior binocular field might then also be reduced. 



b- binocular field; m, m- monocular fields; u, u- residual, uniocular fields; x, x- anterior 



and posterior blind areas. 



such conditions. No wonder, then, that so many fishes have been able 

 to establish themselves and survive in lightless caves. 



The optical density of water has interesting consequences upon aquatic 

 vision, particularly upward through the surface and into the air. Re- 

 peated allusion has already been made to the fact that the corneal tissue 

 has about the same refractive index as water, so that the cornea is in 

 effect optically absent under water, and the first bending of incoming 

 light-rays takes place at the surface of the lens. This requires the lens to 

 bulge far through the pupil if that aperture is not to limit greatly the 

 visual field (Fig. 105b, p. 261); and the lens must project from the sur- 



