ANALYSIS OF LIGHT. 



directed to the highest part being the most refrangible ; the rays 

 directed to the intermediate parts having intermediate degrees of 

 refrangibility. 



3. Rays which are differently refrangible are also differently 

 coloured. 



4. The least refrangible rays composing solar light are the 

 red rays, which compose the lowest division E of the spectrum. 

 But these red rays are not all equally refrangible, nor are they 

 precisely of the same colour. The most refrangible red rays are 

 those which are deflected to the lowest point of the red space R, 

 and the least refrangible are those which are directed to the 

 point where the red melts into the orange. Between these there 

 are an infinite number of red rays having intermediate degrees 

 of refrangibility. The colour of the red rays varies with their 

 refrangibility, the most intense red being that of rays whose 

 refrangibility is intermediate between those of the extreme rays of 

 the red space. 



The same observations will be applicable to rays of all the other 

 colours. 



5. Each of these components of solar light having a different 

 refrangibility will have for each transparent substance a different 

 index of refraction. Thus the index of refraction of the red rays 

 will be less than the index of refraction of the orange rays, and 

 that of these latter will be less than the index of refraction of 

 the yellow rays, and so on ; the index of refraction of violet rays 

 being greater than for any other colour. 



But the rays of each colour being themselves differently refran- 

 gible, according as they fall on different parts of the coloured 

 space, they will, strictly speaking, have different indices of re- 

 fraction. The index of refraction, therefore, of any particular 

 colour must be understood as expressing the index of refraction 

 of the middle or mean ray of that particular colour. Thus, the 

 index of refraction of the red rays will be the index of refraction 

 of the middle ray of the red space ; the index of refraction of the 

 orange rays will be the index of refraction of the middle ray of 

 the orange space ; and so on. 



It must not, however, be supposed that a pencil of solar light 

 consists of separate and distinct rays of different colours which 

 form the spectrum, so that it might be possible by any mechanical 

 division of such a pencil to resolve it into such rays. Each indi- 

 vidual ray of such a pencil is composed of all the rays of the 

 spectrum, just as the gases oxygen and hydrogen, which are the 

 chemical constituents of water, enter into the composition of each 

 particle of that liquid, no matter how minute it be. 



19. As the solar light is resolved by the prism into the various 



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