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



About thirty consistent comparisons were made, ranging over the 

 spectrum from G to beyond F, and a curve drawn on the plan of 

 fig. 1. I do not give the complete curve, because I hope before 

 long to complete and confirm the observations ; but the following 

 numbers will give an idea of the result : — 



c. 



D. 



b 3 . 



F. 



25 



40 



63 . 



80 from fig. 1 



25 



41 



71 



90 observed. 



The upper line gives the theoretical intensities for the fixed 

 lines C, D, b 3 , F, while the lower gives the observed ratios be- 

 tween the lights (sky and diffused sunlight), the two sets of 

 numbers being made to agree at C. Considering the difficulties 

 and uncertainties of the case, the two curves agree very well ; 

 and it should be noticed that the sky compared with diffused light 

 was even bluer than theory makes it, on the supposition that the 

 diffused light through the paper may be taken as similar to that 

 whose scattering illuminates the sky. It is possible that the 

 paper was slightly yellow ; or the cause may Ik in the yellowness 

 of sunlight as it reaches us compared with the colour it possesses 

 in the upper regions of the atmosphere. It would be a mistake 

 to lay any great stress on the observations in their present incom- 

 plete form ; but at any rate they show that a colour more or less 

 like that of the sky would result from taking the elements of 

 white light in quantities proportional to X -4 . I do not know 

 how it may strike others ; but individually I was not prepared 

 for so great a difference as the observations show, the ratio for 

 F being more than three times as great as for C. 



There is one point in which our calculations do not exactly 

 meet the case of the sky. In the experiments with precipitated 

 clouds the total quantity of light scattered is quite insignificant 

 compared with the incident beam ; but it is by no means so 

 clear that the same is the case with the sky. Each particle is 

 thus struck, not only by the direct light of the sun, but also by 

 that scattered from others. It does not seem that the chromatic 

 effects would be much affected by this consideration ; but it is 

 worth notice that the conclusion as to complete polarization per- 

 pendicular to the incident ray would have to be modified. To 

 see this, imagine, as before, the light (unpolarized) incident 

 along Y upon a particle ; we have seen that the ray dif- 

 fracted along X contains no vibration parallel to Y. By the 

 aid, however, of another particle P in the xy plane such a vibra- 

 tion may be communicated to it ; for in the ray diffracted from 

 P to there is a component vibration in the xy plane perpendi- 

 cular to P 0, which, when again diffracted along X, will give 

 a component parallel to Y. This is perhaps the explanation 



