[ 237 ] 



Some ^bougbt6 on %\Qbt 



By F. W. Sutcliffe, F.R.M.S. 



WHAT is light ? This question has often been asked, and a 

 variety of theories propagated by way of answer. 



For a long time it was generally supposed that light was an 

 emanation of particles from some luminous body or bodies, and 

 that just as the minute particles of some gaseous body are carried 

 along until they reach the organ of smell, producing in that organ 

 the peculiar sensation of smell, so the philosophers of old 

 imagined that minute particles emanated from a variety of special 

 bodies, and were carried along until they eventually came into 

 contact with the eye, thereby producing the sensation and idea of 

 light. 



But this is an exploded theory, and the philosophers of to-day 

 are inclined to the opinion, that the phenomenon of light is pro- 

 duced solely and simply by undulations in, or vibrations of an 

 elastic air. Every luminous body is regarded as the seat of a 

 motion which is in some peculiar way transmitted to our optic 

 nerve, arousing in that nerve a sensation of brightness. 



Dr. Lommel puts the matter very clearly in his work on 

 Optics, when he says that there are only two modes by which 

 movement may be propagated from one point to another, the first 

 of which is the immediate transference of motion, in which the 

 body itself, or parts of the body, traverse the space between the 

 two points; the second mode of transference taking place medi- 

 ately through some elastic intervening medium, and in which 

 medium the original motion excites a vibratory movement which 

 is propagated from particle to particle, to a great distance, without 

 any particle of the original body moving from its first position. 



Now, it is undoubtedly admitted, that it is by this latter undu- 

 latory movement that light is spread ; and we find that there are 

 certain peculiarities connected with the action of both light and 

 sound, which go to prove the truth of this assertion. 



Following out certain experiments as directed by Dr. Lommel, 

 we arrive at the curious and startling result that light added to 



