EMISSION THEORY, 



233 



ago traces of the former in the writings of Empedocles. 

 Among tlie 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- 



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

 moved on turning the handle by cranks at their lower end, so ai-ranged 

 that each bull 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 represents a wave with plain 

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



