320 PHYSIOLOGICAL PHYSICS. [Chap. XXVL 



ensues, and it is violet rays that are received on the 

 second screen H. 



Recomposition of white light. If a spec- 

 trum, produced by passing a ray of white light s 



through one prism, be 

 immediately passed 

 through a second 

 prism, in every way 

 the same as the first, 

 but inverted, the re- 

 fraction of the two 



Fig. 148. Kecomposition of White 



Light. prisms is opposite in 



direction, the coloured 



rays are reunited, and a white ray E emerges from 

 the second prism. 



Theory of the spectrum. White light is 

 not, then, simple, but is a compound of various 

 colours. Each colour of the spectrum has its own 

 degree of refrangibility. All the colours are refracted 

 when passed through a prism, though unequally. 

 Thus, red light is refracted to a certain extent, yellow 

 light to a greater extent, violet light most of all. . In 

 a word, the refrangibility increases from red, where 

 it is least, up to violet, where it is greatest. Thus 

 the red end of the spectrum is called the low end, or 

 end of least refrangibility ; while the violet end is 

 the high end, or that of greatest refrangibility, When 

 white light, which is thus a compound of rays of 

 different refrangibilities, is passed through a prism, 

 each ray is refracted according to its own degree, and 

 thus the different colours are separated out and pro- 

 jected on to a screen in the order of their refrangi- 

 bility, the two extremes being red and violet, with 

 the rays of intermediate refrangibilities between them. 



As we have seen, the coloured band produced is 

 called the spectrum, the separation of the different 

 rays being called DISPERSION. 



