CONTEMPORARY ADVANCES IN PHYSICS 65 



by a physicist working with a quite ordinary spectroscope, a quite 

 ordinary source of Hght and some very familiar chemicals, all of which 

 had been available to everyone for at least fifty years. The physicist 

 who thus saw what for half a century the whole world had overlooked 

 was C. V. Raman of Calcutta. 



Within a few months it was observed that what Raman had discov- 

 ered was one of the special cases of a very general principle, which had 

 already been stated almost as clearly as one can state it even today, 

 but had failed of due recognition, evidently for want of some 

 conspicuous example. Other special cases had indeed already been dis- 

 covered among the phenomena of X-rays, but for some reason or other 

 all except one had failed to make the impression which they should 

 have made. Raman's discovery brought the principle sharply into 

 relief. I shall not state it fully at this point; but this is its practical 

 consequence : when light falls upon matter, the scattered rays need not 

 all have the same frequency as the infalling. Most of the scattered 

 light may preserve the initial frequency unchanged as formerly it was 

 supposed that all did; but some part of it may be modified or shifted. 



The present paper is devoted to this principle almost entirely; but 

 to give it the proper background, I must at least mention some of the 

 other aspects of scattering. Light may be scattered by particles of any 

 size, from atoms up to grains of dust or droplets of mist; even the 

 reflection from larger bodies is to be considered as the resultant of 

 light-scattering from the particles of which these are made. Scat- 

 tering by granules or droplets may be analyzed by the electromagnetic 

 theory, the substance of the particle being considered as a medium 

 with a different index of refraction and a different coefficient of absorp- 

 tion from the surrounding air. This however is distinct from the case 

 with which we have now to deal, the scattering of light by molecules 

 or atoms — although the transition from one type to the other should 

 be very interesting and instructive. 



The effect produced by molecules or atoms — it is known as the 

 Tyndall effect, from its earliest thorough student — -is fairly con- 

 spicuous in liquids or crystalline solids, though a very small trace of 

 dust or finely-dispersed precipitate produces a brilliant scattering 

 which completely overwhelms it. In gases it is difficult to see, but not 

 impossible. The blue of the sky is a specific instance which is easy to 

 observe, because the air is so deep. 



The features of the scattered light which are commonly studied — 



I am speaking now of the time when it was assumed that there is no 



change in frequency — are its intensity, its polarization, and a property 



which is rather vaguely known as "coherence." The intensity of the 



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