ETC. 73 



tie to do with the theory of light, except as regards the modi- 

 fications of luminous rays in passing through the refracting 

 media of the eye. It will be sufficient to state that nearly 

 all physicists of the present day agree in accepting what is 

 known as the theory of undulation, rejecting in toto the emis- 

 sion-theory proposed by Newton. Newton supposed that all 

 luminous bodies gave off inconceivably small particles capable 

 of traversing space with wonderful rapidity and of penetrat- 

 ing transparent bodies. This view was at first very generally 

 accepted ; but, as scientific facts accumulated, it was found to 

 be opposed to many optical phenomena, and the theory of 

 undulation now takes its place. It is necessary to the theory 

 of undulation to assume that all space and all transparent 

 bodies are permeated with what has been called a luminif er- 

 ous ether ; and that light is propagated by a vibration or un- 

 dulation of this hypothetical substance. This theory assimi- 

 lates light to sound, in the mechanism of its propagation ; 

 but, in sound, the waves are supposed to be longitudinal, or to 

 follow the line of propagation, while in light the particles are 

 supposed to vibrate transversely, or at right angles to the line 

 of propagation. It must be remembered, however, that the 

 undulatory theory of sound is capable of positive demonstra- 

 tion, and that the propagation of sound by waves can only 

 take place through ponderable matter, the vibrations of which 

 can always be observed; while luminous vibrations involve 

 the existence of an imponderable and purely hypothetical 

 ether. It is possible, indeed, that scientific facts may, in the 

 future, render the existence of such an ether improbable or its 

 supposition unnecessary; but, at present, all we can say is 

 that the theory of luminous undulation is entirely in accord 

 with the optical phenomena that have thus far been rec- 

 ognized. 



The different calculations of physicists with regard to the 

 velocity of light have been remarkably uniform in their re- 

 sults. The lowest calculations put it at about 185,000 miles 

 in a second, and the highest, at about 195,000 miles. The 



