152 The Right Hon. Lord Rayleigh [May 7, 



WEEKLY EVENING MEETING, 



Friday, May 7, 1920. 



Sir James Reid, Bart., G.C.V.O. K.G.B. M.D. LL.D., 



Vice-President, in the Chair. 



The Right Hox. Lord Rayleigh, Sc.D. F.R.S. M.R.T., 

 Professor of Physics, Imperial College of Science. 



The Blue Sky and the Optical Properties of Air. 



Scattering by Small Particles. Polarisation. 



The subject chosen for this evening is one which specially interested 

 my father throughout his career. I shall try to put before you some 

 of his conclusions, and then pass on to more recent developments, 

 in which I have myself had a share. 



Let us begin with one of his experiments which illustrates the 

 accepted theory of the blue sky. We have here a glass tank contain- 

 ing a dilute solution of sodium thiosulphate. A condensed beam 

 from the electric arc traverses it and then falls on a white screen, 

 where it shows the usual white colour. I now add a small quantity 

 of acid, which decomposes the solution with slow precipitation of 

 very finely divided particles of sulphur. As soon as this precipitation 

 begins you see that light is scattered — that is to say, that it is diverted 

 to every side out of the original direction of propagation. Moreover, 

 you will observe that the scattered light is blue. The transmitted 

 beam is robbed of its bluer constituents, and tends to become yellower, 

 as you may see on the screen. 



The light scattered laterally is to be compared to the blue sky ; 

 the yellow transmitted light to the direct light of the setting sun, 

 when it has traversed a great thickness of air. 



As the precipitation goes on, the transmitted light becomes orange, 

 and even red. But the particles of sulphur eventually get bigger, 

 and then give a less pure blue in the lateral direction. We shall 

 have more than enough to occupy us if we confine our attention to 

 the earlier stages, when the particles are small compared with the 

 waves of light. 



A very important property of the scattered light is its polarisation. 

 The vibrations of the scattered light as you have seen it, viewed 

 laterally in the horizontal plane, are almost wholly up and down. 

 No light is emitted which vibrates in the horizontal plane. It is 

 easy for individual observers to verify this with a Nicol's prism held 



