10 THE FORMS OF WATER IN 



light, you have a source of agitation in the vibrating 

 atoms, or smallest particles, of the luminous body ; yon 

 have a vehicle of transmission in a substance which is 

 supposed to fill all space, and to be diffused through the 

 humours of the eye ; and finally, you have the retina, 

 which receives the successive shocks of the waves. 

 These shocks are supposed to produce the sensation of 

 light. 



27. We are here dealing, for the most part, with 

 suppositions and assumptions merely. We have never 

 seen the atoms of a luminous body, nor their motions* 

 We have never seen the medium which transmits their 

 motions, nor the waves of that medium. How, then, do 

 we come to assume their existence ? 



28. Before such an idea could have taken any real 

 root in the human mind, it must have been well disci- 

 plined and prepared by observations and calculations of 

 ordinary wave-motion. It was necessary to know how 

 both water-waves and sound-waves are formed and 

 propagated. It was above all things necessary to know 

 how waves, passing through the same medium, act upon 

 each other. Thus disciplined, the mind was prepared 

 to detect any resemblance presenting itself between the 

 action of light and that of waves. Great classes of 

 optical phenomena accordingly appeared which could 

 be accounted for in the most complete and satisfactory 

 manner by assuming thern to be produced by waves, and 

 which could not be otherwise accounted for. It is 





