now wr: sek. 179 



lishod by proof as positive as science can furnish us. I 

 have endeavored to do this in a paper read before the Philo- 

 sophical Society of this city, in December, 1880. 



It NVituld not be pertinent, at this time, to enter into an 

 elaborate exposition of this theor}'. Let it suffice to say 

 that the two fundamental facts on which it is based are: 

 1st. The evidence which com|)arative anatomy and physi- 

 ology furnish that the eye is but a highly specialized nerve 

 of common sensation. It is a fact which has been thoroughly 

 well established by observation that certain of the lower 

 animals are affected by light over the whole extent of their 

 external surface. As wc ascend higher in the scale of exis- 

 tence the differentiation of function becomes greater, and 

 while the grosser effects of the ether vibration are still felt 

 in the form of heat on the surface of the body, tlu; liner 

 vibrations are appreciated only in certain restricted locali- 

 ties known as eyes; for it is a fact which is disputed, 1 think, 

 by no one that heat and light are both due to vibrations of 

 the same ether, differing in degree onl}^ and not at all in 

 kiml. 



Prof. Preyer, of Jena, has, within the last few months, 

 advanced very similar views, and shows that the sense of 

 heat and the sense of sight bear a very strong analogy to 

 each other. The second fact is the reaction of simple sub- 

 stances to hght, as manifested by a change in their molecu- 

 lar structure. It has long been known that certain colors 

 have greater chemical powers, for instance, than others. 

 Th* so-called actinic power of blue or violet is greater than 

 that of yellow. This is only a short method of stating the 

 fact that the change in the molecidar structure of the afi'ected 

 body is much greater by light rays of short-wave lengths 

 than it is by light rays of longer-wave lengths. The action 

 of light on the metal silenium, and on other substances 

 used in the construction of the photophone, is an example 

 more to the point. Not only is the electric conductability 

 of these substances modified by slight changes in the inten- 

 sity of the light waves, but Prof. Bell has found that this 

 variation in conductability, which only re])resents change 



