ORGANS OF SPECIAL SENSE THE EYE 357 



(1) that the amount of pigment increases when the eye is 

 closed, and decreases when light enters it; (2) that if the eye 

 of an animal be closed, the animal then immediately killed 

 and the eye first opened in a solution of alum in a dark room, 

 the image of the last thing seen by the eye can thus be pre- 

 served in the pigment layer. (Such a picture is called an 

 optogram). A difficulty in the way of accepting this theory 

 is that it supposes the pigment to be constantly broken down 

 by light and again restored. In reading, for example, the 

 shapes of thousands of letters and the spaces between them 

 and between the lines are continually appearing and forming 

 rapidly changing pictures on the retina. In the ordinary use 

 of the eyes, hundreds of different images are being formed arid 

 changed every minute. In order to be continually in con- 

 dition to receive new light rays, i.e. receive new images, the 

 pigment would have to undergo repairs at a rate of which we 

 can hardly conceive. 



The Mechanical Theory. The mechanical theory supposes 

 that light falling on the retina disturbs or shakes the rods and 

 cones, and thus starts in them impulses which travel 

 through their connections to the optic nerve and thence go 

 to the brain. How can this be possible? In answering this 

 question we have to remember that light itself is in the form 

 of very short, wave-like movements. The rapidity of these 

 vibrations is almost beyond our powers of appreciation; but 

 the theory is that they pass into the eye and thus stir the rods 

 and cones to the same rapid movement, the disturbance 

 being interpreted in the brain as "light." 



The main reason for doubting that the effect of light in the 

 eye is so simple is that the light waves take place at such very 

 rapid rates. For instance/ the slowest movements which we 

 interpret as light at all occur at the rate of 107,000,000,000,000 

 per second; the fastest rate which we can perceive at all is 

 40,000,000,000,000,000 per second. Red light vibrates at 

 392,000,000,000,000, and violet at 757,000,000,000,000 per 



