176 ELEMENTARY GENERAL SCIENCE CHAP. 



Refraction is accompanied by Dispersion. In all the cases 

 of refraction which have been considered the phenomena have 

 been described as if all the ether- waves, which are contained in 

 white sunlight, are bent equally, but this is really not so. If 

 the experiments are performed as described, in every case it 

 will be noticed that the image formed by the refracted ray is 

 coloured round its edges. To prevent this complication, and to 

 make the descriptions quite correct, we must suppose that waves 

 of a given length are used, or, as it is called, monochromatic 

 light, such, fpr example, as could be obtained by burning methyl- 

 ated spirit in which common salt is dissolved. 



FIG. 89. To show Principal Focus of a Double Convex Lens. 



The shortest, most rapid waves are bent most ; the slowest, 

 longest waves are bent least. This can be very prettily seen by 

 performing an experiment which was first done by Newton ; 

 improving on his method, however, by using a slit instead of a 

 round hole, through which to let the light pass. 



The Analysis of Light by a Prism. EXPT. 173. Allow 

 sunlight to fall through a slit or hole, Fig. 90, into a darkened 

 room. Just inside the room arrange a prism as shown. The 

 component wave-lengths of the white light will be differently 

 refracted by the prism, the rapid vibrations of the ether 

 which give rise to the sensation of violet will be most bent, 

 the slowest vibrations or the red waves will be bent least, and 

 a band of colour, continuous between these limits and com- 

 prised of the colours of the rainbow, will be formed. 



