280 



NA TURE 



[August 26, 1922 



has progressed, and it is because he is still (and 

 rightly) dissatisfied with the answers given that he 

 continues to advance. 



There may conceivably be a limit to the acquisi- 

 tions and interpretations of a finite mind ; but man 

 is as yet so evidently in his infancy mentally, 

 psychically, and even politically, that we are in no 

 immediate danger of knocking our heads against 

 that possibly predestined barrier to profitable in- 

 quiry. Nor do 1 for one believe that any such barrier 

 exists. There is manifestly an Unknown, but I 

 should hesitate to describe it as the Unknowable. 

 A. Wyatt Tilbv. 



Howstean, Frinton, Essex. 



I am very glad to accept Mr. Tilby's disclaimer. 

 I did indeed associate his evolution of consciousness 

 with a certain theory of " emergence " made famous 

 in an address to the Psychological Section of the 

 British Association last year. I did not mean to 

 suggest that Mr. Tilby's theory was obscurantist or 

 dogmatic. The Reviewer. 



Transparency of Liquids and Colour of the Sea. 



In an earlier note in Nature (Nov. 24, 1921, 

 vol. 108, p. 402) J pointed out that the scattering 

 of light in its passage through a liquid resulting from 

 the local fluctuations of density, the magnitude of 

 which is given by the Einstein-Smoluchowski relation, 

 should enable its transparency to be determined for 

 the parts of the spectrum in which it does not exercise 

 selective absorption. It should be mentioned that 

 in making an experimental test of this point, account 

 has also to be taken of the scattering resulting from 

 the anisotropy of the molecules and that there is 

 an important difference between this and the scatter- 

 ing due to density -fluctuations. The orientation- 

 scattering is almost completely unpolarised and is 

 therefore distributed symmetrically in all directions. 

 The density-scattering is polarised and is twice as 

 intense longitudinally as in a transverse direction. 



The coefficient of extinction resulting from the 

 joint effect of both types of scattering can be calcu- 

 lated theoretically if" the compressibility, refractive 

 index, and the ratio of the components of polarisa- 

 tion in the transversely-scattered light are known. 

 Taking the case of benzene as an example, the co- 

 efficients of extinction calculated for the 5461 and 

 435 s nnes of the mercury spectrum, which fall in 

 regions in which there is no selective absorption, are 

 respectively 0-00022 and 0-00060. These values agree 

 very closely with the recent experimental determina- 

 tions of Martin, and form a striking confirmation of 

 the theory. There is little doubt that the observed 

 transparency of many other liquids will similarly 

 be found to be in agreement with theory when accurate 

 data are available. 



The case of water is of special interest. Of all 

 ordinary liquids it is the one for which the coefficient 

 of scattering is smallest, and is therefore most affected 

 by traces of selective absorption. There is an 

 absorption band which is clearly marked up to 0-5 fj., 

 and it is possible that traces of it extend into the 

 blue region of the spectrum. For the 4358 line, the 

 coefficient of extinction calculated theoretically is 

 0-00006 and Martin's observed value is 0-00012. It 

 seems probable that a little farther out in the violet, 

 the transparency may agree more closely with that 

 derived from the theory of scattering. 



The newer data now available enables a quantita- 

 tive test to be made of the theory put forward by 

 me in a recent paper (Proc. Roy.Soc., April 1922) 

 that the blue colour of the deep sea arises from the 



NO. 2756, VOL. I 10] 



molecular scattering of sunlight in water, the thick- 

 ness of the effective layer being determined by the 

 attenuation of the sun's rays as they penetrate into 

 the liquid. The tentative calculations made in that 

 paper have now been revised. The table shows the 

 theoretical albedo of ocean water expressed in terms 

 of the equivalent scattering by dust-free air at 

 normal temperature and pressure. 



Albedo of Ocean Water. 



It is evident from these figures that the blue of the 

 sea would be much more saturated than the blue of 

 the sky, which is the standard of comparison. The 

 height of the homogeneous atmosphere being 8 

 kilometres, the sea would be about half as bright as 

 the zenith sky on a clear day. This agrees well 

 with the photometric determinations made by 

 Luckiesh during aeroplane flights over deep ocean 

 water in the Atlantic (Astrophysical Journal, vol. 

 49, 1919, p. 129). Luckiesh makes it clear that 

 the greater part of the observed luminosity of water 

 viewed perpendicularly really arises from light 

 diffused upwards from within the water. His 

 determinations thus appear to furnish a quantitative 

 proof of the theory which attributes the colour of 

 the deep sea to molecular scattering of light. 



C. V. Raman. 



210 Bowbazar Street, Calcutta. 



Telescopic Observation of Atmospheric 

 Turbulence. 



In his recent contribution to meteorology, " Physics 

 of the Air " (U.S. Weather Bureau, Washington), 

 Prof. Humphreys refers, in chapters 11, 12, and 14, 

 under the general headings of "Wind Layers" 

 (p. 219), "Wind Billows" (p. 221), "Barometric 

 Ripples" (p. 228), and "Special Cloud Forms" 

 (p. 296), to the demonstration by Helmholtz (trans- 

 lated by Cleveland Abbe, " Mechanics of the Earth's 

 Atmosphere," Smithsonian Institution, 1891) that 

 "adjacent layers of air ^differ abruptly from each 

 other in temperature, humidity, and density, and 

 therefore may and often do glide over each other 

 with ... a wave-producing effect." Prof. Hum- 

 phreys proceeds, of course, to associate these demon- 

 strations with the problems of atmospheric turbulence. 



May I be allowed to point out, however, that it 

 is not the case, as stated by Prof. Humphreys (p. 219), 

 that " these air waves are seen only when the con- 

 ditions of humidity at the interface are . . . just 

 right " for the condensation of visible clouds — I 

 speak from the experience of personal observations 

 covering, intermittently, a period of upwards of 

 twenty years. These " Wind Billows " or " Air 

 Waves " of Helmholtz's demonstrations are always 

 readily visible in the absence of clouds. The various 

 directions of their flowings, and the order of their 

 temporary stratification, are accurately legible by the 

 method I employ of a projected telescopic image of 

 the sun for the purpose of their observation. The 

 " cautious aviator," instead of succumbing to the 

 idea that he needs must fly in the face of " unknown 

 danger," should know that the early stages of turbu- 

 lence are — if only the sun is unclouded — at all times 

 conspicuously and spontaneously recognisable by this 

 very simple method of observation. 



Catharine O. Stevens. 



The Plain, Boar's Hill, Oxford, August 9. 



