DISPERSION OF LIGHT. 481 



when the angle is only 10. But the substance of the 

 prism has also an essential influence upon the extent of 

 the coloured space. A water prism with a refracting 

 angle of 15 produces about the same deviation as a 

 glass prism with a refracting angle of 10 ; an object 

 seen through either prism appears equally displaced 

 from its true position, but the coloured fringe produced 

 by the water prism is less broad than that produced 

 by the glass prism : the difference in the deviation of 

 the various colours is greater in the glass prism than 

 in the water prism, or, as it is termed, glass effects 

 a greater ' dispersion of light ' than water. Even 

 different kinds of glass produce different amounts of 

 dispersion; thus flint-glass possesses nearly twice the 

 4 dispersive power ' of crown-glass. A liquid substance 

 called Carbon Disulpliide has a still greater dispersive 

 power than flint-glass ; a prism filled with this sub- 

 stance, and having a refracting angle of 45 or 50, is 

 therefore especially well adapted for experiments on 

 dispersion. 



A suitable form of such a prism is shown in fig. 

 277, A ; a section of it is represented in B. A portion 

 of a lamp cylinder c c c, cut obliquely at both ends, is 

 closed by glass plates pp r an( i PP& an d has on the 

 upper side an aperture, to be closed afterwards, for 

 pouring in the liquid. 



If such a prism, which produces a deviation of from 

 32 to 38, be held with its refracting edge downwards 

 in the path of the rays of sunlight which fall through a 

 window, the rays will be refracted upwards, and will 

 produce on the opposite wall not a white spot with a 

 coloured fringe, but a band of colours in which no longer 



I I 



