Absorption Spectra of Dilute Solutions. 125 



and the part reflected will be 



If the light is incident 011 the plate from the liquid instead of from 

 air, the part transmitted will be the same, while that reflected will 

 be 



+ 



I 



Now consider in the same way the layer of liquid as being a plate, 

 the end surfaces of which have the property of transmitting a frac- 

 tion t of the light incident upon them, and of reflecting parts r or r', 

 according as the light is incident externally or internally, and which 

 further transmits a part, b c, of the light which passes through its 

 whole length once. Take b to be the transmission coefficient for the 

 solvent, and c that for the dissolved substance. Then the part of the 

 light transmitted by the whole system is 



................. (1). 



T is the quantity which is directly determined ; from it we have to 

 calculate c. b, t, and r are determined by preliminary experiments. 

 The values of t and r depend on the refractive* indices of the glass 

 plates and of the solutions. The solutions used were all so dilute 

 that no appreciable error is made by using the same values for t and 

 r' as are found for pure water. 



Having found the value of c, the extinction coefficient e l5 for the 

 dissolved body, is obtained by means of the relation log c = ^. 



The calculation of c from equation (1) may be much simplified in 

 the following way. We may write equation (1) : 



For the glass plates used the greatest possible value of (6c)V 2 was 



rp 



0-0023. So that approximately (6c) 3 == , from which 



u 



T 



= - log c = log * 3 6-log T - log ( i-r' a Y . . . (2). 



Log fb only depends on the wave-length of the light used, and is most 



r' 2 

 conveniently obtained from a curve drawn once for all. Also is 



