﻿116 The Hon. J. W. Strutt on the Light from the Sky, 



the fraction representing the transmission of A may be, its 36th 

 power will give the transmission of R. To take an instance, if 



Fig. 2. 



*9 of the ray A gets through, only "018 of R would be able to 

 penetrate. For the rays of still higher refrangibility, which 

 Professor Stokes found abundant in the electric light but missing 

 in the solar rays, the fraction would be smaller still ; but I am 

 not aware of any measurements of smaller w T ave-length on which 

 to found a calculation. 



We have hitherto supposed that the light scattered by the 

 finely divided matter reaches the eye without modification, and 

 we have taken no account of any change in the composition of 

 the primary light before diffraction. If x be the total length of 

 the path of the ray through the turbid medium, we may express 

 the quality of the light in terms of x ; for it makes no difference 

 whether the lateral leakage takes place before diffraction or after. 

 In fact 



Ioc\- 4 e-^~ 4 *, 



an expression which shows that I vanishes for very small as well 

 as for very large values of X, while for some definite value (say A) 

 it rises to a maximum (I ). Expressing I in terms of I and A, 

 we have 



A 4 2 _: 

 X 4 



'A*-; 



from which we may fall back on our original law by supposing 

 A indefinitely small, and replacing A 4 I by a finite constant. 

 An approximate idea of the character of these lights may be ob- 

 tained by subtracting the successive curves of fig. 2. Thus the 

 difference of the curves marked 2 and 4 represents a light having 

 its maximum brightness (of course relatively to the primary 

 light) in the blue-green portion of the spectrum. I find by cal- 

 culation that, if the maximum intensity be at b and be taken as 

 unity, the intensities at G and C are given by the numbers *713, 

 •710 respectively. The colour would be greenish ; but whether 



