328 



Dr. T. Ewan on the Absorption-Spectra of 

 Table V. 



Region in 





(1-pO 



a-P') 



Spectrum. 





for 7 "64 cent i in. 



for 22-87 ceniam. 



X 627-7— X 599-4 



0-00108 



0-981 





599-4— 574-2 



0-00073 



0-987 





574-2— 551-6 



000039 



0-993 



0-980 



551-6— 534-1 



0-00026 



0-996 



0-986 



534-1— 518-2 



0-00019 



0997 



0-990 



518-2- 502-3 



0-00018 



0-997 



0-990 



502-3— 488-3 



0-00017 



0-998 



0-991 



488-3— 474-6 



000016 





0-992 



474-6— 462-8 



0-00015 





0-993 



The quantity (1— p) is the fraction of the incident light 

 which has escaped being absorbed by the dissolved substance, 

 and is dependent on the length of layer and concentration 

 of solution. In order to obtain comparable numbers, the 

 extinction-coefficients and absorption-ratios have been calcu- 

 lated from the values of (1 —p). A few words on the con- 

 nexion of these quantities with one another are perhaps not 

 altogether superfluous. 



According to Lambert's law we have 



P=I*» (1) 



where V is the light transmitted by a layer of thickness n, of 

 a substance the coefficient of transmission of which is a. I is 

 the original intensity of the light. 



Bun sen and Roscoe * define the extinction-coefficient as the 

 reciprocal of that length of layer of a solution which will 

 reduce the intensity of light by absorption to yo °f its original 



value. 



Substituting, therefore, in equation (1), 1'=^, 1 = 1, and 

 I i 



n=-, we get To = « e ' 



or e= — log a (2) 



From equation (1) we have also 



— n log a= log I— log F. 



Substituting the value of —log a and putting 1 = 1, we get 



c= log I'. 

 n 



V is the quantity which before has been called (1— p) % 



Pogg. Ann. ci. p. 235 (1857). 



