THK EYE AND THE PHYSIOLOGY OK VISION. 531 



SOME OF THE ELEMENTARY PHYSICAL CONCEPTIONS OF LIGHT 



NECESSARY TO A PROPER UNDERSTANDING OF THE 



PHYSIOLOGY OF THE EYE. 



The eye is a complicated piece of apparatus especially 

 designed to translate light impressions into nervous impres- 

 sions. Its construction is for a specific purpose and its 

 anatomy dependent upon the physical properties of the 

 light it is to translate. It is, therefore, entirely impossible 

 to understand the raison d^etre of the eye's structure with- 

 out having an elementary conception at least of some of the 

 physical properties of light. 



1. The Nature of Light. For a long time light was 

 looked upon as small particles of matter projected in straight 

 lines from a luminous body. These little particles passed 

 into the eye, and there in some way produced the sensation 

 of light. A lamp, therefore, would be something like a 

 spherical Gatling-gun throwing its missiles in every con- 

 ceivable direction at a very great velocity. These particles 

 were looked upon as exceedingly small and light, and able 

 to penetrate even such hard substances as glass. 



This material projection of light had soon to be aban- 

 doned as thoroughly unable to explain all of the phenomena 

 at hand. As a result of more exacting study there was ad- 

 vanced soon what is now familiarly known as the undulatory 

 theory of light, the theory which conceives of light as oscil- 

 lations or waves of some suitable medium. This medium 

 is the ether, possibly a hypothetical substance, and yet one 

 to the actual existence of which we are driven by the ne- 

 cessity of the mind. 



This ether is a very light imponderable medium offering 

 little if any resistance to objects moving through it. It not 

 only pervades all space, but seems also to pervade many 

 solid objects, such as glass and others which we are wont 

 to call transparent. Rays of light are conceived as vibra- 

 tions passing through this medium, in which the actual 

 particles of the medium move but little out of their place, 



