478 



NA TURE 



[April 15, 1922 



classical theory of light and the theory in which 

 light is regarded as made up of single quanta of 

 energy emitted discontinuously from luminous atoms. 

 Fig. I (reproduced from the paper) illustrates the 

 proposed experiment. K is a stream of canal rays, 

 Li is a focussing lens, S is a screen containing a slit 

 which serves to isolate a definite pencil of light, and 

 the lens Lg renders the emergent beam parallel. 

 The emergent pencil is observed through a telescope 

 focussed for infinity, so that the image of the slit 

 in the screen S would be seen sharply focussed in 

 the field of view. Since the atoms in the canal rays 

 emitting light are in motion, the Doppler efiEect 

 comes into evidence, and the rays proceeding at any 

 instant from individual luminous atoms in different 

 directions should, according to the wave-theory of 

 light, be of different frequencies. Einstein suggests 

 that the rays passing through the slit S and in- 

 cident on the upper and lower parts of the lens Lg 

 should consequently be of different frequencies. If, 

 therefore, a layer of a dispersing medium such as 

 carbon disulphide be placed between the lens Lg 

 and the observing telescope, the different rays would 

 travel through it with different velocities. Hence 

 the wave-front should suffer an aberration and the 

 image of the slit seen in the focal plane should shift 

 through an extent proportionate to the thickness of 

 the dispersing layer introduced. Einstein conceives 

 that according to the quantum theory of light, on 

 the other hand, such displacement should not occur, 

 and he believes that the proposed arrangement 

 furnishes an experimentum cruets to decide between 

 the rival theories. 



I wish here to direct attention to a fallacy which 

 is present in Einstein's reasoning and invalidates it. 

 It is clear that in the proposed experiment what 

 would be observed are not the moving luminous 

 atoms but the fixed edges of the illuminated slit in 

 S, and it is easily shown that even according to the 

 principles of the wave-theory no aberration of the 

 image of the latter could be expected. To make 

 this evident we may conceive the slit to be extremely 

 narrow, or in the alternative, if it be wide, regard it 

 as divided up into a large number of very narrow 

 elements each of which, according to Huyghens's 

 principle, would operate as a secondary source of 

 light. The light from any small portion of the lens 

 Lj arriving at the slit would spread out by diffraction 

 in all directions in the form of cylindrical waves, so 

 that the waves reaching Lg would consist everywhere 

 of superposed wave-fronts of all the frequencies 

 reaching the slit, and not, as Einstein supposes, of 

 different frequencies at different points of Lg. The 

 waves diverging from S would thus pass through 

 L2 and the dispersing medium behind it according 

 to the ordinary laws of geometrical optics, and no 

 shift or aberration of the image of the slit would 

 occur. The error in Einstein's reasoning lies in his 

 having ignored the vitally important part which 

 diffraction plays, according to the wave-theory of 

 light, in the theory of the formation of images of 

 illuminated apertures by optical instruments. 



C. V. Raman. 



210 Bowbazaar Street, Calcutta, March 16, 1922. 



The Weathering of Mortar. 



Mr. Richardson's letter (March 9, p. 310) anent 

 the above calls, I think, for some further remarks. 

 My observations never led me to conclude that the 

 growth of moss was in any way responsible for the 

 development of the ridge-and-ring markings upon 



NO. 2737, VOL. 109] 



the surfaces of old and exposed mortar. I have 

 seldom seen moss growing thereon, but lichen is 

 very often present, covering the whole surface, and 

 not limited to the linear concavities. There is a 

 row of fishermen's cottages at St. Ives, Cornwall, 

 fully exposed to the sea, and the old mortar in the 

 walls shows the markings in unusual perfection, but 

 there is no evidence that moss ever grew upon it. 



In 1896 I suggested that the phenomenon was a 

 physical effect due to the expansion and contraction 

 — or perhaps the former only — set up by alternations 

 of temperature in a substance like mortar. Strains 

 and stresses along lines of least resistance would 

 tend to destroy compaction along such lines, render- 

 ing them more liable to be deepened by atmospheric 

 erosion. Possibly the expansion and contraction in 

 the stones themselves may also affect the stability 

 of the mortar. 



The accompanying photograph (Fig. i), taken 

 many years ago, shows some ridge-and-ringed mortar 

 at the base of an old wall near Corfe. The wall 

 faced south, and was built of Purbeck limestone. 

 During its construction some thin pieces were placed 

 vertically between the two masses of mortar. The 

 penny fixes the dimensions of the mural components. 

 The structure here was very conspicuous, but, with 

 lapse of time, the photograph has faded somewhat. 

 C. Carus-Wilson. 



March 17, 1922. 



MetchnikofE (Me^nikov) and Russian Science in 1883. 



I have read with the greatest interest the review of 

 the " Life of Elie Metchnikofi " published in Nature 

 of February 9. In Nature of November 17, 1921, I 

 gave an account of the present state of science in 

 Russia and its " prole tarisation," and I beg to be 

 permitted to say a few words on the state of science 

 under Russian absolutism. 



According to the above review, the book referred to 

 says that the government of the university of Odessa 

 became more and more reactionary ; but it was not 

 for political reasons that Metchnikoff left the uni- 

 versity — the reasons were " scientific." 



Following an invitation, I took part in the Congress 



