Blue Glass on the Prismatic Spectrum. 



121 



+ A, 



(1.) 



Firr. 1. 



that iff*, be the index of refraction, say for the ordinary glass 



2 It 

 composing a prism, and — the time of vibration, we have for 



f** an expression of the form 

 iit2 = Ai - A 



Now suppose that we con- 

 struct the curve whose ab- 

 scissa is n and ordinate ju,, by 

 means of this equation; let 

 OX, OY (fig. I.) be the axes, M 

 and A L the curve. If we 

 wish to find the colour corre- 

 sponding to any particular 

 refractive index, we have only 

 to measure O M equal to that, 

 refractive index, and draw 

 M P parallel to O X ; then 

 M P is the corresponding 

 value of w, and determines the colour required. 



37. Now I think the appearances, which it is our object to 

 explain, are most probably caused by the peculiar form of the 

 curve A L, and the peculiar degree of absorption exercised by 

 the blue glass corresponding to different values of 7i (for we 

 have shown (art. 21.) that the index of absorption, like the 

 index of refraction, is a function of 7z). 



Suppose that the curve A L is of the shape represented in 

 Fig. 2. 



O 



X 



X 



fig. 2 ; let A be the point corresponding to the extreme red, 

 Phil. Mag. S. 3. Vol. 26. No. 171. Feb. 1845. K 



