132 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



a grating, in the manner we have been describing, is in 

 their wave-lengths, or what comes to the same thing, in 

 their times of vibration. The diversity of effects pro- 

 duced depends on the quality of the surface on which 

 they fall. If on a dark surface, and the more so in pro- 

 portion to its blackness, they engender heat ; if on the 

 retina, they are interpreted by the mind as light ; if on 

 photographic preparations, they produce decompositions, 

 designated actinic effects. 



Heat, light, actinism, are, then, not natural principles 

 existing independently of each other, but effects arising 

 in bodies from the reception of motions in the ether, 

 motions which differ from each other in their rapidity. 

 Of those that the eye can take cognizance of, the most 

 rapid impart to the mind the sensation of violet light, 

 the slowest the sensation of red, and intermediate ones 

 the intermediate optical tints. Colors, like light itself, 

 are nothing existing exteriorly. They are merely mental 

 interpretations of modes of motion in the ether, and in 

 this they represent musical sounds, which exist only as 

 interpretations by the mind of waves in the air. 



