382 Lord Rayleigh on the Transmission of Light through 



conditions as 19 X 10 18 per cub. centim. If we nse this value 

 of », we find 



# = 8*3xl0 6 cm. = 83 kilometres, 



as the distance through which light must pass through air 

 at atmospheric pressure before its intensity is reduced in the 

 ratio of 2*7 : 1. 



Although Mount Everest appears fairly bright at 100 miles 

 distance as seen from the neighbourhood of Darjeeling, we can- 

 not suppose that the atmosphere is ns transparent as is 

 implied in the above numbers ; and of course this is not to 

 be expected, since there is certainly suspended matter to be 

 reckoned with. Perhaps the best data for a comparison are 

 those afforded by the varying brightness of stars at various 

 altitudes. Bouguer and others estimate about *8 for the 

 transmission of light through the entire atmosphere from a 

 star in the zenith. This corresponds to 8' 3 kilometres of air 

 at standard pressure. At this rate the transmission through 

 83 kilometres would be ("8) 10 , or "11, instead of 1/e or *37. 

 It appears then that the nctual transmission through 83 kilo- 

 metres is only about 3 times less than that calculated (with 

 the above value of n) from molecular diffraction without any 

 allowance for foreign matter at all. And we may conclude 

 that the light scattered from the molecules would suffice to 

 give us a blue sky, not so very greatly darker than that 

 actually enjoyed. 



If n be regarded as altogether unknown, we may reverse 

 our argument, and we then arrive at the conclusion that n 

 cannot be greatly less than was estimated by Maxwell. A 

 lower limit for n, say 7 X 10 18 per cubic centimetre, is some- 

 what sharply indicated. For a still smaller value, or rather 

 the increased individual efficacy which according to the 

 observed refraction would be its accompaniment, must lead to 

 a less degree of transparency than is actually found. When 

 we take into account the known presence of foreign matter, 

 we shall probably see no ground for any reduction of 

 Maxwell's number. 



The results which we have obtained are based upon (14), 

 and are as true as the theories from which that equation was 

 derived. In the electromagnetic theory we have treated the 

 molecules as spherical continuous bodies differing from the 

 rest of the medium merely in the value of their dielectric 

 constant. If we abandon the restriction as to sphericity, the 

 results will be modified in a manner that cannot be precisely 

 defined until the shape is specified. On the whole, however, it 

 does not appear probable that this consideration would greatly 

 affect the calculation as to transparency, since the particles 



