the Weights of Atoms. 299 



matic analysis of the light observed, or something equivalent 

 to it, is necessary. This was done by Rayleigh himself for 

 the blue light of the sky actually before he had worked out 

 his dynamical theory. He compared the prismatic spectrum 

 of light from the zenith with that of sunlight diffused through 

 white paper ; and by aid of a curve drawn from about thirty 

 comparisons ranging over the spectrum from C to beyond F r 

 found the following results for four different wave-lengths. 





C. 



D. 



b\ 



F. 



Wave-length . 



656*2 



589-2 



517-3 



486-2 



Calculated . 



1 



1-54 



2-52 



3-34 



Observed . 



1 



1-64 



2-84 



3-60 



On these he makes the following remarks: — " 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 lie 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 obser- 



" vations in their present incomplete form ; but at any rate 



u they show that a colour more or less like that of the sky 



" would result from taking the elements of white light in 



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" quantities proportional to A," 4 . 1 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." For 



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myself I thoroughly agree with this last sentence of Rayleigh's. 

 There can be no doubt of the trustworthiness of his obser- 

 vational results ; but it seems to me most probable, or almost 

 certain, that the yellowness or orange-colour of the sunlight 

 seen through the paper, caused by larger absorption of green, 

 blue, and violet rays, may explain the extreme relative richness 

 in green, blue, and violet rays which the results show for the 

 zenith blue sky observed. 



§ 77. An elaborate series of researches on the blue of 

 the sky on twenty-two days from July 1900 to February 

 1901 is described in a very interesting paper, " Ricerche 

 sul Bleu del Cielo," a dissertation presented to the Royal 

 University of Rome by Dr. Giuseppe Zettwuch, as a thesis 

 for his degree of Doctor in Physics. In these researches 

 prismatically analysed light from the sky was compared with 



X2 



