LIGHT. 91 



colours an oblong space of a white card e /, properly placed to 

 receive it. The solid wedge of glass is called a prism, and the 

 oblong coloured image on the card, the solar spectrum. Newton 

 counted seven bands of different colours in the spectrum, which, 

 as they succeed each other from the upper part of the spectrum 

 represented in the figure, are violet, indigo, blue, green, yellow, 

 orange and red. The beam of light admitted by the aperture in 

 the window-shutter has been separated in passing through the 

 prism into rays of different colours, and this separation ob- 

 viously depends upon the rays being unequally refrangible. The 

 blue rays are more considerably refracted or deflected out of 

 their course, in passing through the glass, than the yellow 

 rays, arid the yellow rays than the red. Hence the violet end 

 is spoken of as the most refrangible, and the red as the least 

 refrangible end of the spectrum. 



The coloured bands of the spectrum differ in width, and are 

 shaded into each other ; and it is not to be supposed that there 

 are really rays of seven different colours. Sir D. Brewster 

 has established, in a recent analysis of solar light, that there are 

 rays of three colours only, blue, yellow and red, which were 

 well known to artists to be the three primary colours, of which 

 all others are compounded. 



A certain quantity of white light, and a portion of each of the 

 primary rays, may be found at every point from the top to the 

 bottom of the spectrum. But each of the primary rays predo- 

 minates at a particular part of the spectrum. This point is, for 

 the blue rays, near the top of the spectrum ; for the yellow rays, 

 somewhat below the middle ; and for the red rays, near the 

 bottom of the spectrum. Hence, there exist rays of each colour 

 of every degree of refrangibility ; but the great proportion of 

 R ed the yellow rays is more refrangible 



spectrum J J . 



than the red, and the great proportion 

 of the blue more refrangible than either 

 the yellow or red. The compound spec- 

 trum which we observe, is in fact pro- 

 duced by the superposition of three 

 simple spectra, a blue, a yellow, and a 

 red spectrum. The distribution of the 

 rays in each of these simple spectra is 

 represented by the shading in the an- 

 nexed figures. Of the seven different coloured bands into 



