DISPERSION. 39 



in con-eluding that solar light is compound, and consists of 

 an infinite number of simple rays, which are permanent in 

 their own nature, but differ from one another both in their 

 colour and refrangibility. 



(48) The following experiment may be considered as 

 removing all doubt on this subject. Close behind the prism 

 is placed a board, perforated with a small aperture, through 

 which the refracted light is permitted to pass. This light is 

 then received on a second board, similarly perforated, at a 

 considerable distance from the first ; so that a small portion 

 of the light of the spectrum is suffered to pass through the 

 aperture in the second board, the rest being intercepted. Close 

 behind this aperture a second prism is fixed, having its refract- 

 ing edge parallel to that of the first. The first prism being then 

 turned slowly round its axis, the light of the spectrum will 

 move up and down on the second board, and the differently- 

 coloured rays will be successively transmitted through the se- 

 cond aperture, and be refracted by the prism behind it. If 

 then the places of these twice-refracted rays on the screen be 

 noted, the red will be found to be lowest, the violet highest, 

 and the intermediate colours in the same order as they are in 

 the spectrum. Here, on account of the unchanged position of 

 the two apertures, all the rays are necessarily incident upon 

 the second prism at the same angle ; and yet some of them 

 are more refracted, and others less, in the same proportion as 

 by the first prism. 



We conclude, then, that the peculiar colour and refran- 

 gibility belonging to each kind of homogeneous light, are 

 permanent* and original affections, and are not generated 



* Professor Stokes has recently discovered that the refrangihility of light 

 does undergo alteration in certain cases, some bodies possessing the property of 

 lowering the refrangibility of the incident light that is, of emitting rays of a 



