CHAPTER XXVI. 



ANALYSIS OF LIGHT : COLOUR. 



A PRISM has a remarkable effect on white light as 

 the result of its refractive properties. 



The spectrum. If a ray of sunlight s entering 

 a room through a narrow slit in a shutter, be caused 



O 



to pass through a prism A interposed in its path, as 

 shown in the figure, a band of colour will be thrown 



O 



on to a screen placed beyond the shutter. Seven 



Fig- 147 The Spectrum. 



colours will be made out in regular order from below 

 upwards, as follows : red, orange, yellow, green, 

 blue, indigo, violet. No sharp line of demarcation is 

 visible between different colours, but one merges 

 gradually into the succeeding colour. The band of 

 colours is called the spectrum. Suppose ail the 

 colours except the violet v at the high end of the spec- 

 trum, to be caught on the screen E, but the violet to 

 be permitted to pass the screen, and be intercepted 

 by a second prism B, it is found that the violet rays 

 are again refracted, but no further decomposition 



