PATH OF LIGHT THKOUGH A PJRISM 9 



measurement it will be found that E F makes, with the normal at 

 F, an angle 9' of 25^, and for the blue ray an angle <" of 26J. 



It should be remembered, however, that if the refracting angle 

 of the prism is known, there is no necessity for this measurement, 

 because it is always the difference between this and the angle of 

 refraction before determined, thus 50 24^= 25 V>. 



R 



B~ C ^V 



FIG. 5. Diagram of deviation of luminous ray by a prism. 



This ray E F now becomes the incident ray on the surface A C ; 

 and as the angle it makes with the normal at F is known, and as 

 the refractive indices remain the same, we can, by (problem) v. 2, 

 find the angles of refraction for each colour. 



If we take red light : 



, the angle of refraction =47 (found by table). 



If we take blue light : 



sn 



-75 sin 26J 175 x '442 



__ L = - T __ = . m . 



<j), the angle of refraction = 50 J (found by table). 



This dispersion can now be represented in the diagram, seeing 

 that it amounts to 3|. 



In optics it is convenient to use an expression to measure the 

 dispersive power of diaphanous substances, which does not depend 

 on the refracting angle of the prism employed. Further, in order 

 that various substances may be compared, their dispersive powers 

 are all measured with reference to a certain selected ray. (For this 

 purpose the bisection of the D or sodium lines is the point in the 

 spectrum often chosen.) 



In the crown and flint glasses mentioned on page 4 the dispersion 

 between the lines C and F, in the spectrum, referred to the bisection 

 of the sodium lines D, is as follows. Crown glass : refractive index 

 bisection of lines D, l'5179=/u; line F, 1'52395=//; line C, 

 l'51535=/x // . Then the dispersive power to 



1-52395 -1-51 535 -0086 



=<01661 - 



ufuf' 



_l 1-5179 -1 



-5T79 



