488 Mr. E. H. Cook on the 



and many inactive bodies, that all the former possess one or 

 more asymmetrical carbon-atoms — an asymmetrical carbon- 

 atom being one which is united with unlike simple or com- 

 pound radicals. Here, then, we have a mass of evidence 

 which unquestionably shows that optical rotation depends 

 upon the molecules of which the bodies are built up. But this 

 rotation is produced by the passage of a ray of light through 

 the body. Therefore the passage of a ray of light -through a 

 body depends upon, and is affected by, the structure of the 

 molecules of that body. Hence the molecules of the body are 

 the vehicles which transmit the light. Assuming the inter- 

 molecular existence of the luminiferous aether, these experi- 

 ments upon optical rotation cannot be explained without 

 making assumptions which are unreasonable. If this aether 

 does transmit the vibrations of light, then the above conclu- 

 sions regarding the molecules and atoms are not just, because 

 we now have to deal, not with the molecules, but with the 

 spaces between these molecules. It would thus appear that 

 the study of optical rotation offers very strong evidence in 

 favour of the molecular theory of vibration, while it throws 

 great difficulty in the acceptance of the aether theory. 



4. The Photophone. 



The difficulty in explaining the decrease in the electrical 

 resistance of selenium on exposure to light was briefly noticed 

 in my previous paper. Since that time, however, the brilliant 

 success which has attended the experiments of Prof. Graham 

 Bell and Mr. Sumner Tainter has given the subject fresh 

 interest. From being an interesting scientific curiosity, these 

 researches have shown it to be but a result of a general law. 

 It will therefore be of advantage to notice briefly the chief 

 results which the various investigators have obtained in this 

 particular subject. 



In addition to the discovery of the fundamental fact, Mr. 

 Willoughby Smith* proved that solar light has a much more 

 powerful effect upon selenium than light from any other source. 

 Lieutenant Sale|, who was the next investigator to publish 

 his results, came to the conclusion that the effect is not due to 

 any change in the temperature of the selenium bar, and also 

 that the effect is produced instantaneously. He concluded that 

 there exists a power in rays nearly coincident with the heat- 

 rays of low refrangibility " of altering instantaneously the 



* Journal of the Society of Telegraph Engineers, vol. vi. 

 t Proc. Roy. Soc. vol. xxi. p. 283. 



