Chapter 1 

 LIGHT AND ITS PERCEPTION 



The principal means by which most animals are made aware of their 

 surroundings, and changes in these surroundings, is the reflection or 

 emission of light toward them by external objects and the reception of 

 this light by special organs which we term photoreceptors. The more 

 complicated of these photoreceptors are called eyes, though it is not 

 complexity, as such, which governs the applicability of that special term. 

 We say that the function of the eye is vision, but since all photoreception 

 is not vision and not all photoreceptors are eyes, we must consider these 

 broader and narrower terms before delving into our subject proper — 

 the structure and variations of vertebrate eyes and their relation to the 

 ways of life of their possessors. 



Light may best be defined, for our purposes here, as a rhythmic eman- 

 ation of energy whose rhythm-frequency or pitch falls within definite 

 limits, outside of which are the higher or lower frequencies of radio, 

 cosmic, X-, and other rays. Visible light thus forms a circumscribed 

 band of frequencies to which the eye happens to be sensitive and which, 

 compared with all forms of radiant energy in general, is like a single 

 octave toward the high-pitched end of the scale of a piano (see Table I) . 

 It contains only a small fraction of the total amount of energy given off 

 by the sun, and sunlight in turn forms only a portion of the 'grand 

 spectrum' of radiant energy. Like other forms of radiant energy, light 

 in its ultimate units can vary in but simple ways — in speed, in frequency, 

 and in intensity. But natural lights and illuminations are complex mix- 

 tures of these variations, and make possible the infinite variety of nature's 

 pictures, varying in tone or shading (owing to combinations of inten- 

 sities) and in color or hue (owing to combinations of frequencies) , 



We have been discussing light as an objective physical entity; but, just 

 as there would be no sound if a tree were to fall with no one to hear it, 

 so also there would be no light in the physiological sense if there were no 

 photoreceptor upon which it impinged. In this other sense light is a 

 sensation, an experience in consciousness. Like other such experiences, 

 it may be evoked by a limited number of causes (other than actual 



