EMISSION THEORY. 238 
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- 
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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 represents a wave with plain 
vibrations, in the latter, with elliptic or circular vibrations. 
