256 THE SPECIAL SENSES 



the hairs of sensitive cells lying in the wall. Nervous 

 impulses are thereby started which result in our hav- 

 ing a sense of the direction and rapidity of the move- 

 ment of the head. 



THE SENSE OF SIGHT 



Nature of light. Light, upon which our sensations of 

 sight depend, resembles sound in that it is a form of 

 vibration. The vibration, however, is not of the air 

 but of the ether, a gas so extremely thin that it pene- 

 trates bodies and fills all space. Light itself in all its 

 variety of color consists merely of vibrations of ether 

 which pass from luminous objects at varying rates of 

 speed. Ordinarily, it is a mixture of many different 

 rates which blend together to give us white light. When 

 one rate of vibration prevails in the light which reaches 

 the eye, we perceive the color characteristic of that 

 rate, as red when the rate is three hundred and ninety- 

 two trillions of vibrations per second, or violet when 

 it is seven hundred and fifty-seven trillions. 1 



General structure. The mechanism of the eye con- 

 sists essentially of two parts. The first part is an optical 

 instrument corresponding to a camera with lens and 

 screen. Rays of light from external objects are focused 

 upon a screen, the retina, at the back of each eye. The 

 second part is a nervous mechanism in the retina by 

 which the rays of light thus focussed give rise to nervous 

 impulses, and these in turn to our sensations of sight. 



1 The colors lying between the two extremes of red and violet are 

 orange-red, orange, orange-yellow, yellow, yellow-green, green, green- 

 blue, blue and blue-violet. Vibrations which are slower than those 

 giving red or faster than those giving violet are not perceived by 

 the eye, although experiments have proved that they exist. 



