120 THE ANATOMY OF SCIENCE 



trivances of this sort. Light from a source S, 

 let us say an incandescent filament, is reflected 

 from a mirror AA' and falls upon the screen 

 CD. But the light from the same source falling 

 upon the mirror BB', which makes a slight 

 angle with AA', also falls in the same region of 

 the screen, where it alternately diminishes and 

 enhances the light from the other mirror, thus 

 producing upon the screen a series of dark and 

 bright bands. 



This is but one of numerous phenomena 

 which are so varied in their manifestation that 

 we must have some sympathy for the student 

 who wrote in his examination, "The interfer- 

 ence of light, as I understand it, is very little 

 understood." Yet all these phenomena follow 

 so directly from the undulatory theory of light, 

 and have been reduced to such simple princi- 

 ples, and can be predicted with such accuracy, 

 that there is no body of established thought 

 wliich we should abandon with greater reluc- 

 tance than the theory that light is in some way 

 associated with some sort of waves or periodic 

 disturbances proceeding outward in spherical 

 symmetry from the source of light. 



The older theory that these waves were me- 

 chanical disturbances in an elastic ether has, 



