CHAPTER XXV 



Vision — introduction 



H. K. HARTLINE | Rockefeller Institute for Medical Research, New York City 



WE ARE INDEED 'CHILDREN OF THE SUN'. The ultimate 



dependence of li\ing organisms on solar enersy is 

 probably one reason why animals came to evolve 

 highly specialized sensory receptors for exploiting the 

 sun's radiations. And since the green plants utilize 

 wavelengths in that part of the solar spectrum reach- 

 ing the earth's surface in greatest amount, it is not 

 surprising that the receptors evolved by plant-eating 

 animals and their predators also should operate in 

 roughly the same range of wavelengths, which we in 

 consequence call visible light. 



Wavelengths of visible light are small compared 

 with the size of the bodies of most animals and of 

 many significant objects in their surroundings. Hence 

 light reflected, scattered and absorbed in varying 

 degrees by objects in an animal's environment makes 

 an ideal physical agent for providing information 

 about that environment. This possibility has been 

 exploited by nearly all animal forms to a remarkable 

 degree. 



It is appropriate that a neurophysiologist taking up 

 the study of vision should begin with a consideration 

 of the extraordinary diversity in which eyes have 

 evolved in 'lower' animal forms. An intimation of 

 this diversity is given in the Milnes' chapter on in- 

 vertebrate photoreceptors. Missing from this hand- 

 book is a comparable discussion of the eyes of the 

 vertebrates, which, though of but a single type, 

 nevertheless show a great variety of ingenious adap- 

 tations to meet special needs. Fortunately, this de- 

 ficiency is easily remedied by reference to Walls' 

 excellent and highly readable hook. The Vertebrate 



EyeCn')- 



Of the great variety of visual organs that the animal 

 kingdom has developed, many are no mean per- 

 formers. Our own eyes, for all their defects, are 

 excellent physical instruments, all the more remark- 



able for being constructed, by embryological magic, 

 out of gristle and jelly. Yet man need not think he 

 has the best of all possible eyes. He terms the short 

 wavelengths 'ultraviolet', but they are visible to at 

 least some insects. Polarized light elicits the entoptic 

 phenomenon known as ' Haidinger's brushes', the 

 orientation of which reveals the direction of the 

 plane of the light's polarization. A few individuals 

 are said to be able to perceive these brushes when 

 viewing the blue sky with unaided vision. But as far 

 as is known to the author, no race of men has utilized 

 this as a sky compass, comparable to the use made 

 by many of the arthropods of their ability to sense 

 the plane of polarization of sky light. Man's eyes are 

 remarkably sensitive; they can detect approximately 

 loo quanta, but many nocturnal vertebrates un- 

 doubtedly have a lower effective threshold. Our 

 visual acuity is surpassed by that of some other ani- 

 mals, especially the acuity of birds of prey. Yet for 

 all of this, man is at no very great disadvantage 

 merelv because the visual apparatus of other animals 

 surpasses his own in some special directions. His 

 visual equipment is not over-specialized, and it does 

 many things very well. 



Interest in light and vision dates back to antiquity. 

 Nearly everyone has heard of the quaint idea of the 

 Greeks, that light is an intangible ray-like emanation 

 from the eye itself, exploring tactually the surround- 

 ings. (Indeed, if we were to assume that sensation 

 could result only if such emanation were absorbed by 

 what we term luminous objects, this idea would not 

 be easy to disprove; in physics the optical principle 

 of the reversibility of path is often invoked in theo- 

 retical discussions.) With a history of many interesting 

 misconceptions, a sound understanding of the nature 

 of light and the structure and function of the eye 

 gradually emerged. By the time of Kepler many of 



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