4-88 BAKON VON WREDE ON THE ABSORPTION OF LIGHT 



A(l -r'Y 



^/ 



9 A' 

 1 - 2 r'2 COS 2 TT _ + r'* 



L.M. 



where 



= \/'- 



W^ 



2 r- cos z TT — + 

 \ 



M = \/ 1 -2r'2cos2 7r — +/-'4 



111 the same manner the results of the three retarding causes will be 

 (1 -rr-(l -r'y (I -r"y- 



where 



A" 



L = 



M 



N = a/(i — 2r"2cos27r— + r"A. 



From these equations it will be evident that in general A' or A" 



must attain a partial maximum or a partial minimum as often as one 



26 b' 



of the angles 2 tt — , 2 tt — , &c. has completed an entire periphery; 

 X X 



that is to say, thei-e must originate in the resulting spectrum as many 

 absorptions as would have originated in the sum of each single spec- 

 trum. It is therefore easy to account for that which must arise 

 from two or more retardations, in the manner which I will now show 

 by an example. If we convey light into a vessel into which iodine has 

 been brought, and which is then gradually heated so that the iodic 

 gas may increase slowly in intensity, the piiaenomena of absorption 

 take place in the manner and order following : as soon as so much 

 iodic gas is disengaged that the vessel receives a slightly reddish tint, 

 we perceive in the blue light, or rather at the limit between the blue 

 and the violet, some slender pale black stripes. The dark stripes be- 

 come blacker the more the intensity of the colour increases, antl at the 

 .-ame time we perceive more dark stripes. With increasing intensity of 

 the colour of tho iodic gas the light stripes in the blue commence gra- 

 dually to decrease in strength, till at last a complete absorption of the 

 blue end of the spectrum takes place. In proportion as the entire ab- 



