November J 7» 192^] 



NATURE 



3^7 



experiments, using the K lirjes froni tungsten (effec- 

 tive A = o-i96 A.) reflected from rock-salt, and 

 measured the absorption coefficient of the secondary 

 radiation excited by these rays in paraffin. The ab- 

 sorption coefficient of these rays was found to be con- 

 siderably greater, by about 52 per cent, at 90° and 

 21 per cent, at 30°, than that of the beam incident on 

 the paraffin. 



In order to compare my results with those of Mr. 

 Plimpton, a molybdenum Coolidge tube was then sub- 

 stituted for the tungsten one, and the Ka line 

 (A = 0708 A.) was employed. An increase in the ab- 

 sorption coefficient of the secondary rays excited in 

 paraffin was again observed, though it amounted to 

 only 29 per cent, at 90° and only 6± 1-2 per cent, at 20° 

 with the primary beam. 



The softening thus observed when reflected X-rays 

 are scattered is substantially the same as that found 

 when unreflected rays of the same hardness are em- 

 ployed. Mr. Plimpton's negative result is apparently 

 due to the fact that his experiment was performed 

 under unfavourable conditions of wave-length and 

 scattering angle. The conclusion seems necessary, 

 therefore, that the softening of secondar}' X-rays is 

 due, not to the process of scattering, but to the excita- 

 tion of a fluorescent radiation in the radiator. 



Arthur H. Comptov. 



Washington University, St. Louis, U.S.A. 



The Colour of the Sea. 



The view has been expressed that "' the much- 

 admired dark blue of the deep sea has nothing to 

 do with the colour of water, but is simpfy the blue of 

 the sk}' seen by reflection " (Rayleigh's Scientific 

 Papers," vol. 5, p. 540, and Nature, vol. 83, p. 48, 

 19 10). Whether this is really true is shown to be 

 questionable by a simple mode of observation used by 

 the present writer, in which surface-reflection is 

 eliminated, and the other factors remain the same. The 

 method is to view the surface of the water through a 

 Nicol's prism, which may for convenience be mounted 

 at one end of a tube so that it can be turned about 

 its axis and pointed in any direction. Observing a 

 tolerably smooth patch of water with this held in 

 front of the eye at approximately the polarising angle 

 with the surface of the sea, the reflection of the sky 

 may be quenched by a suitable orientation of the 

 Nicol. Then again, the sky-light on a clear day in 

 certain directions is itself strongly polarised, and an 

 observer standing with his back to the sun when it 

 is fairly high up and viewing the sea will find the light 

 reflected at all incidences sufficiently well polarised 

 to enable it to be weakened or nearly suppressed by 

 the aid of a Nicol. 



Observations made in this way in the deeper waters 

 of the Mediterranean and Red Seas showed that the 

 colour, so far from being impoverished by suppres- 

 sion of sky-reflection, was wonderfully improved 

 thereby. A similar effect was noticed, though some- 

 what less conspicuously, in the Arabian Sea. It was 

 abundantly clear from" the observations that the blue 

 colour of the deep sea is a distinct phenomenon in 

 itself, and not merely an effect due to reflected sky- 

 light. When the surface-reflections are suppressed 

 the hue of the water is of such fullness and satura- 

 tion that the bluest sky in comparison with it seems 

 a dull grey. 



By putting a slit at one end of the tube and a 

 grating over the Nicol in front of the eye, the spec- 

 trum of the light from the water can be examined. 

 It was found to exhibit a concentration of energv in 

 the region of shorter wave-lengths far more marked 

 than with the bluest sky-light, 

 NO. 2716, VOL. 108] 



Even when the .§ky was completely, overcast the 

 blue of the water cbutd be obseri/ed with the aid of 

 a Nicol. It was then a deeper and fuller blue tharr^ 

 ever, but of greatly enfeebled intensity. The altered 

 appearance of the sea under a leaden sky must thus 

 be attributed to the fact that the clouds screen the 

 water from the sun's rays rather than to the inci- 

 dental circumstance that they obscure the blue light of 

 the sky. 



Perhaps the most interesting effect observed was 

 that the colour of the water (as seen with the Nicol 

 held at the polarising angle to the surface of the 

 water and quenching the surface-reflection) varied 

 with the azimuth of observation relatively to the plane 

 of incidence of the sun's rays on the water. When 

 the plane of observation and the plane of incidence 

 were the same, and the observer had his back to the 

 sun and looked down into the water, the colour was 

 a brilliant, but comparatively lighter, blue. As the 

 plane of observation is swung round the colour 

 becomes a deeper and darker blue, and at the same 

 time decreases in intensity, until finally when the plane 

 of observation has swung through nearly 180° the water 

 appears very dark and of a colour approaching indigo. 

 Both the colour and the intensity also varied with the 

 altitude of the sun. 



The dependence of the colour on the azimuth of 

 observation cannot be explained on a simple absorp- 

 tion theory, and must evidently be regarded as a 

 diffraction effect arising from the passage of the light 

 through the water. Looking dcwn into the water 

 with a Nicol in front of the eye to cut off the surface- 

 reflections, the track of the sun's rays could be seen 

 entering the water and appearing by virtue of perspec- 

 tive to converge to a point at a considerable depth 

 inside it. The question is : What is it that diffracts 

 the light and makes its passage visible ? An interest- 

 ing possibility that should be considered in this con- 

 nection is that the diffracting particles may, at least 

 in part, be the molecules of the water themselves. As 

 a rough estimate, it was thought that the tracks 

 could be seen to a depth of 100 metres, and that the 

 intensity of the light was about one-sixth of that of 

 the light of the sky from the zenith. If we assume 

 that clear water, owing to its molecular structure, is 

 capable of scattering light eight times as strongly as 

 dust-free air at atmospheric pressure, it is dear that 

 the major part of the observed effect may arise in 

 this way. 



It is useful to remember that the reflecting power 

 of water at normal incidence is quite small Conly 

 2 per cent.), and becomes large only for ver}- oblique 

 reflection. It is only when the water is quite smooth 

 and is viewed in a direction nearly parallel to the 

 surface that the reflected sky-light overpowers the 

 light emerging from within the water. In other 

 cases the latter has a chance of asserting itself. 



C. V. Raman. 



S.S. Narkuuda, Bombay Harbour, 

 September 26. 



The " Proletarisation of Science " in Russia. 



Dr. H. Lyster Jamesox asks in Nature of Septem- 

 ber 29, p. 147, for an account of the constructive 

 elements of the "proletarisation of science " in Russia, 

 and seems to praise the effort of the Soviet Govern- 

 ment to bring the fundamental conclusions of scientific 

 thought within the reach of the "proletariat " bv 

 editing a whole series of elementary text-books of 

 natural science. 



A Russian university professor, whose friendship I 

 have enjoyed for more than twenty-five years, who 

 has just escaped from the "Bolshevik Paradise," gave 



