October 14, 1922] 



NA TURE 



5°5 



absolute criterion of magnitude, but systems of refer- 

 ence are not even relatively in relations of magnitude 

 to one another. It is only for the observer in a system 

 of reference that there is a relation of magnitude within 

 the system and that the system itself has relations of 

 magnitude to other systems. Into whatever system 

 an observer passes he carries into it his own constant 

 norm of magnitude and he does not have to submit to 

 the dimensions which the new system imposes on him. 

 It is this aspect of the principle of relativity which has 

 seemed to the present writer to require a philosophical 

 principle like that of the Leibnizian monad to give it 

 full expression. It is not enough to return to the 

 mathematical principle of Descartes's mechanism. 

 Mathematics and physics alike rest ultimately on the 

 experience of active subjects, and this is why experi- 

 mental tests are relevant. The monadic conception 

 derives new meaning from the theory of reality as 

 psychical duration, the concept which Bergson has 

 made a new possession of human thought. 



II. W'ildon Carr. 



The Molecular Scattering of Light. 

 Molecular Diffraction of Light. By Prof. C. V. Raman. 

 Pp. x + 103. (Calcutta: University of Calcutta, 

 1922.) 



READERS of Nature are already familiar with 

 the important work which Prof. C. V. Raman 

 has been carrying out in connexion with the scattering 

 of light by small particles, for many of his results have 

 been announced in these columns. In a small volume 

 published by the University of Calcutta he has reviewed 

 the present position of the subject of molecular diffrac- 

 tion of light, and has discussed the theory in a com- 

 prehensive survey which includes the case of gases, 

 vapours, liquids, crystals, and amorphous solids. 



Lord Rayleigh was the first to indicate the principles 

 on which the problem may be handled, and he obtained 

 a relation between the scattering power of the molecules 

 of a gas, their number per unit volume, and the refrac- 

 tivity of the medium. As the energy scattered must 

 be derived from the primary beam, the intensity of the 

 latter must suffer an attenuation as it passes through 

 the medium, and an expression can be derived for the 

 attenuation coefficient. Prof. Raman discusses some 

 criticisms of the theory and concludes that the principle 

 of random phase which is assumed in the argument is 

 justified, provided there exists the random distribution 

 of the molecules which is required by Boyle's law. The 

 ultimate justification of the principle rests on the com- 

 plete non-uniformity in the spatial distribution of the 

 molecules in so far as very small volume elements are 

 concerned. 



The first successful attempt to observe the scattering 

 NO. 2763, VOL. I 10] 



of light by dust-free air in the laboratory was made by 

 Cabannes in 191 5. Experimental work of great 

 interest has been carried out by Prof. R. J. Strutt (the 

 present Lord Rayleigh), who obtained the remarkable 

 result that, in many gases, the scattered light is only 

 partially polarised. This may be explained as due to 

 the lack of symmetry of the molecules, and may furnish 

 valuable information with regard to molecular con^ 

 figuration. 



To the late Lord Rayleigh we owe the brilliant 

 estion that the scattering of light by the molecules 

 of air accounted in large measure both for the blue light 

 of the sky and the observed degree of transparency of 

 the atmosphere. Recent observations, principally at 

 the Observatory on Mount Wilson, have confirmed the 

 theory and have furnished a value for Avogadro's 

 constant which is practically identical with that deduced 

 from Millikan's measurements of the electronic charge. 

 Prof. Raman has made observations on the polarisation 

 of skylight on Mount Dodabetta (8750 feet above sea 

 level) in the Nilgiris. As is well known, dust and haze 

 arc largely confined to the lower levels of the atmo- 

 sphere. The influence of secondary scattering may be 

 reduced very considerably by using a deep red filter, 

 and allowance can be made for the effect of earthshine. 

 The weaker component of polarisation was found to 

 have 13 per cent, of the intensity of the stronger com- 

 ponent. Only 4 per cent., however, was ascribable to 

 molecular anisotropy, a result in good agreement with 

 the latest laboratory measurements. 



The principle of random phase on which Rayleigh's 

 theory depends is not applicable in the case of highly 

 condensed media such as dense vapours, liquids, and 

 solids. In liquids, we may apply the theory developed 

 by Einstein and Smoluchowski, in which scattering is 

 considered not as due to individual particles but to 

 small local variations of density arising from the heat 

 movements of the molecules. A formula is obtained 

 showing how the scattering power of a fluid is related 

 to its refractivity. It is worthy of notice that the 

 scattering power is proportional to the absolute 

 temperature and to the compressibility of the liquid. 

 When corrected for the effect of molecular anisotropy, 

 the formula gives results in fair agreement with observa- 

 tions in non-fluorescent liquids, and it reduces auto- 

 matically to the Rayleigh formula in the case of gaseous 

 media. But, surprisingly enough, the law seems to 

 break down in the case of gases under high pressure. 

 Prof. Raman makes the interesting suggestion that this 

 failure may mean that the continuous wave theory of 

 light does not strictly represent the facts, and that we 

 may perhaps find here experimental support for 

 Einstein's conception that light itself consists of 

 quantum units. 



Q I 



