THE EYE AS AN OPTICAL INSTRUMENT. 519 



The shorter the oscillation periods of light-rays the more 

 they are deviated by refraction. Hence mixed light when 



fi" B 



FIG. 146. Diagram illustrating the dispersion of mixed light by a prism. -~ 



sent through a prism is spread out, and decomposed into- its 

 simple constituents. For let a x (Fig. 146) be a ray of mixed 

 light composed of a set of short and a set of long ethereal 

 waves. When it falls on the surface A B of the prism, that 

 portion which enters will be refracted towards the normal 

 E D, but the short waves more than the longer. Hence the 

 former will take the direction x y, and the latter the direc- 

 tion x z. On emerging from the prism both rays will again 

 be refracted, but now from the normals Fy and G z, since 

 the light is passing from a more to a less refracting medium. 

 Again the ray x y, made up of shorter waves, will be most 

 deviated, as in the direction y v, and the long w r aves less, in 

 the direction z r. If a screen were put at S S f , we would re- 

 ceive on it at separate points, v and r, the two simple lights 

 which were mixed together in the compound incident ray 

 a x. Such a separation of light-rays is called dispersion. 



Ordinary white light, such as that of the sun, is composed 

 of ethereal vibrations of every rate, mixed together. When 

 such light is sent through a prism it gives a continuous band 

 of light-rays, known as the solar spectrum,, reaching from the 

 least refracted to the most refracted and shortest waves. The 

 exceptions to this statement due to Frauenhofer's lines (see 

 Physics) are unessential for our present purpose. All of the 

 simple lights into which the compound solar light is thus 



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