1883.] Mr Glazebrook, On a Spectrophotometer. 307 



and if G be the concentration of the solution, we can shew that 

 C=AE, A being a constant for the medium. 



Hence C= — log ( -,, 



m ° \T 



Thus C = -loi 



m & \1 - kj 



If now we are working with a solution contained in a glass 

 vessel we must remember that much of the loss of light is due 

 to reflexion at the faces of the containing vessel. To eliminate 

 this strictly we should require to know the refractive indices of 

 the glass and solution for the light in question, and then determine 

 by means of Fresnel's formulas the loss due to reflexion. The loss 

 due to absorption will be found by subtracting this from the total 

 loss, and it is the value of h thus obtained which we must use in 

 the above formula. In practice this will be inconvenient, and a 

 result which, when the absorption is considerable, will be near the 

 truth may be obtained as follows. 



Take a similar glass cell and fill it with water or some other 

 medium which absorbs the light but little, while its index of 

 refraction does not differ greatly from the solution whose absorption 

 coefficient or concentration is required, place it in the path of the 

 light, and observe the intensity. Let it be I X and let k 1 be the 

 corresponding value of k, 



then I t = I (1 - jy. 



Now insert the absorbing solution and observe again. Suppose 

 we find . I 2 =I(l-k 2 ), 



k 2 is the loss per unit intensity due to absorption and reflexion 

 combined. 



Since the refractive indices of the two media used are nearly 

 the same the amount of loss due to reflexion will be the same in 

 the two cases. Thus, if h denote the loss per unit intensity due to 

 absorption only, h = A' 2 — h x approximately. 



Hence k = ' 



and we get finally 



tan 2 6 X - tan 2 6 2 

 tan 2 6 



A , / tan 2 6 



C = — be 



m b Vtan 2 6 - tan 2 6 X + tan 2 2 

 if we neglect the loss due to reflexion 6 X = 0. 



Thus to use the instrument to measure the amount of concen- 

 tration of a solution, we must observe 6 the position of the Nicol 



