The Senses and the "Nervous System 83 



out of the eye, and in the processes of death and cooking has 

 lost its transparency. 



The eye, in both man and fish, is filled with transparent 

 substances. The light, passing from air into these substances, 

 is refracted and forms an image on our retina, just as it forms 

 an image after passing through the lens of a camera. But in 

 the case of the fish, the light passes not from air into the 

 lens, but from water into the lens. It is obvious that light 

 passing from water into water will not be refracted. The 

 substance inside the eye is of almost the same density as 

 water. Therefore, in passing from water into the eye light 

 undergoes much less refraction than in passing from air into 

 the eye. This is why our vision is blurred when we open our 

 eyes under water. Our lens mechanisms, adequate for form- 

 ing an image from light-rays passing into the dense sub- 

 stance of the eye from air, are not sufficiently curved to bend 

 the rays into a sharp image when they pass into the dense 

 substance of the eye from the scarcely less dense water. To 

 overcome the difficulty, the fish curves the lens more. He 

 goes the limit in degree of curvature, and uses a spherical 

 lens, but even at that some authorities doubt whether he 

 succeeds in bringing the light to a proper focus and in pro- 

 ducing a really clear picture on his retina. And for the same 

 reason that our vision is blurred when our eyes are under 

 water, we are justified in assuming that the fish cannot see 

 clearly when his eyes are in the air. His lens mechanism is 

 designed to handle light coming through water, not light 

 coming through air. 



To accommodate our eyes to varying distances — in other 

 words, to bring the rays of light from any object to a focus 

 on the retina, no matter how far away the object — we are 

 able to change the curvature of our lens. For long range we 

 make it flatter, for short range more curved. But the fish 

 needs all the curvature possible, all the time. If he flattened 



