210 RECENT PROGRESS OF THE THEORY OF VISION. 



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 produce 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 ra- 

 diation 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 processes 

 they may be subjected, it is impossible to weaken their illumi- 

 nating power without at the same time, and in the same degree, 

 diminishing their heating and their chemical action. Whatever 

 produces an undulatory movement 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, accord- 

 ing to the length of the waves. The undulations 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 have the smallest wave-length. This series of 

 colours is universally known in the rainbow. We also see it if we 

 look towards the light through a glass prism, and a diamond 

 sparkles with hues which follow in the same order. In passing 

 through transparent prisms, the primitive beam of white light, 

 which consists of a multitude of rays of various colour and 

 various wave-length, is decomposed by the different degree of re- 

 fraction of its several parts, referred to in the last essay ; and 

 thus each of its component hues appears separately. These 



