238 RECENT PROGRESS OF THE THEORY OF VISION. 



also generally regarded as invisible, and are known as the 

 dark chemical rays. 



Between the very long and the very short waves of 

 ether there are waves of intermediate length, which 

 strongly affect the eye, but do not essentially differ in 

 any other physical property from the dark rays of heat 

 and the dark chemical rays. The distinction between the 

 visible and invisible rays depends only on the different 

 length of their waves and the different physical relations 

 which result therefrom. We call these middle rays Light, 

 because they alone illuminate our eyes. 



When we consider the heating property of these rays 

 we also call them luminous heat ; and because they pro- 

 duce such a very different impression on our skin and on 

 our eyes, heat was universally considered as an entirely 

 different kind of radiation from light, until about thirty 

 years ago. But both kinds of radiation are inseparable 

 from one another in the illuminating rays of the sun ; 

 indeed, the most careful recent investigations prove that 

 they are precisely identical. To whatever optical pro- 

 cesses they may be subjected, it is impossible to weaken 

 their illuminating power without at the same time, and 

 in the same degree, diminishing their heating and their 

 chemical action. Whatever produces an undulatory move- 

 ment of ether, of course produces thereby all the effects 

 of the undulation, whether light, or heat, or fluorescence, 

 or chemical change. 



Those undulations which strongly affect our eyes, and 



'which we call light, excite the impression of different 



colours, according to the length of the waves. The un- 



f dulations with the longest waves appear to us red ; and 



as the length of the waves gradually diminishes they 



seem to be golden-yellow, yellow, green, blue, violet, the 



last colour being that of the illuminating rays which 



