HOW WK sEi:. 167 



Applying this principle to the other, Young supposed 

 that the light force was due to an undulatory motion set up 

 in it by the vibrations of the molecules of luminous bodies- 

 It is now a pretty well demonstrated fact that when bodies 

 get into the condition known as "hot," a change takes place 

 in the ultimate i>articles of the matter comjKjsing them. 

 They are set in vibration. The more heated they become 

 the more extensive will these molecular vibrations be; and 

 as the space occupied by the molecules in a state of vibra- 

 tion is greater than when they are at rest, the substance 

 when heated is larger than when cool. The motion of these 

 ultimate molecules is communicated to the ether by which 

 they are surrounded, and it is set in undulatory motion, 

 just as waves are produced on the surface of water when a 

 body is let fall upon it, which disturbs the relation of its 

 molecules. As the motion of the molecules of the heated 

 body is regular, the wave motion communicated to the sur- 

 rounding ether will also be regular, that is, the waves will 

 follow each other at regular intervals of time. 



Up to this point everything must be taken for granted, 

 for, as I have said, not one of the accepted facts is capable 

 of experimental demonstration. Accepting this much, how- 

 ever, we can deduce certain laws which are capable of sub- 

 stantiation by ex])eriment, and by them we are able to ex- 

 plain all or nearly all the phenomena of light and vision. 

 Though we have shown you that light is only a form of 

 force — a mode of motion — it will be more convenient to 

 speak of it as a Thing, without reference to the ether or 

 wave motion. Thus, wlien we say that light travels at the 

 rate of 1SG,00U miles per second, we mean that the motion 

 communicated by the luminous body to the ether is felt 

 180,000 miles away at the end of a second of time. There 

 are a few fixed laws governing the action of ligiit, to which 

 we briefly call your attention, because on them dci)ends all 

 our knowledge of the physiology of vision. 



In the first place, we know that light moves in straight 

 lines, and never turns out of its course unless it meets with 

 some substance having the property of deflecting it. There 



