EMISSION THEORY. 



233 



ago traces of the former in the writings of Empedocles. 

 Among the moderns I can cite among its adherents, Kep- 

 ler, Newton, and Laplace. The system of waves does 

 not reckon less illustrious partisans ; Aristotle, Descartes, 

 Hooke, Huyghens, Euler, adopted it. Such names on 

 either side render a choice difficult, if in a matter of sci- 



the reader to a very simple machine, represented in the annexed figure 

 contrived by the translator, which exhibits a set of white balls, repre- 







D' 



O 



O 



R 



R' 



" 



O 



n< 



D 



D" 



senting the molecules of ether: these are attached to rods, which are 

 moved on turning the handle by cranks at their lower end, so arranged 

 that each ball is in succession raised or lowered nearly in a straight 

 line; so that they follow each other in the form of a wave. When the 

 bar supporting the rings through which the rods pass, is lowered, the 

 balls no longer move up and down in straight lines, but describe each a, 

 kind of oval curve, which becomes more rounded the lower the bar is 

 placed. In the former case the machine represeuts a wave with plain 

 vibrations, in the latter, with elliptic or circular vibrations. 



