376 Lord Rayleigh on the Transmission of Light through 



question whether the light from the sky can be explained by 

 diffraction from the molecules of air themselves, or whether it 

 is necessary to appeal to suspended particles composed of 

 foreign matter, solid or liquid. It will appear, I think, that 

 even in the absence of foreign particles we should still have a 

 blue sky *. 



The calculations of the present paper are not needed in 

 order to explain the general character of the effects produced. 

 In the earliest of those above referred to I illustrated by 

 curves the gradual reddening of the transmitted light by 



* My attention was specially directed to tins question a long while ao'o 

 by Maxwell in a letter which I may be pardoned for reproducing here. 

 Under date Aug. 28, 1873, he wrote :— 



" I have left your papers on the light of the sky, &c. at Cambridge, 

 and it would take me, even if I had them, some time to get them assimi- 

 lated sufficiently to answer the following question, which I think will 

 involve less expense to the energy of the race if you stick the data into 

 your formula and send me the result 



" Suppose that there are N spheres of density p and diameter s in unit 

 of volume of the medium. Find the index of refraction of the compound 

 medium and the coefficient of extinction of light passing through it. 



"The object of the enquiry is, of course, to obtain data about the size 

 of the molecules of air. Perhaps it may lead also to data involving the 

 density of the aether. The following quantities are known, being com- 

 binations of the three unknowns, 



M = mass of molecule of hydrogen ; 



N= number of molecules of any gas in a cubic centimetre at G° C. 



and 760 B. 

 s = diameter of molecule in any gas: — 



Known Combinations. 

 MN = density. 

 Ms 2 from diffusion or viscosity. 



Conjectural Combination. 



— - = density of molecule. 



" If you can give us (i.) the quantity of light scattered in a given 

 direction by a stratum of a certain density and thickness ; (ii.) the 

 quantity cut out of the direct ray ; and (iii.) the effect of the molecules 

 on the index of refraction, which I think ought to come out easily, we 

 might get a little more information about these little bodies. 



" You will see by ' Nature,' Aug. 14, 1873, that I make the diameter 

 of molecules about ^Vo °f a wave-length. 



" The enquiry into scattering must begin by accounting for the great 

 observed transparency of air. I suppose we have no numerical data 

 about its absorption. 



"But the index of refraction can be numerically determined, though 

 the observation is of a delicate kind, and a comparison of the result 

 with the dynamical theory may lead to some new information." 



Subsequently he wrote, " Your letter of Nov. 17 quite accounts for the 

 observed transparency of any gas." So far as I remember, my argument 

 was of a general character onfy. 



