Marcii.io, 19 io] 



NATURE 



49 



F"urther observations on greater depths of sea-water 

 would be desirable. A naval son informs me that off the 

 coast of Greece a plate lying in 6 fathoms of water looked 

 decidedly blue, although the sky was a dirty grey. 1 have 

 doubts whether this would be generally the case in the 

 Mediterranean ; the green due to moderate thicknesses 

 seems too decided. 



Of natural fresh waters that I have tried, none was 

 better than that from a spring in my own garden. This 

 water is hard, but bright and clear, and it shows a 

 greenish-blue, barely distinguishable from that of the Capri 

 and Suez water. Distillation does not improve the blue. 

 Neither did other treatments do any good, such, for 

 example, as partial precipitation of the lime with alkali, 

 or passage of ozone with the idea of oxidising humus. 

 Wishing to try water of high chemical purity, I obtained — 

 through the. kind offices of Sir J. Dewar — water twice 

 distilled from alkaline permanganate, and condensed in 

 contact with silver, but the colour was no bluer. In the 

 light of this evidence I can hardly avoid the conclusion 

 that the blueness of water in lengths of 4 metres has been 

 exaggerated, especially by Spring, although I have no 

 reason to doubt that a fully developed blue may be 

 obtained at much greater thicknesses. I should suppose 

 that sufHcient care has not been taken to start with white 

 light. It may be recalled that overcast days are not so 

 common in some parts of the world as in England. 



A third possible cause of apparent blueness of the sea 

 must also be mentioned. If a liquid is not absolutely 

 clear, but contains in suspension very minute particles, it 

 will disperse light of a blue character. Although, un- 

 doubtedly, this cause must operate to some extent, I have 

 seen no reason to think that it is important ; but the 

 existence of three possible causes of blueness complicates 

 the interpretation of the phenomena. Hitherto observers 

 have not been sufficiently upon their guard to distinguish 

 blueness having its origin in the sky from blueness fairly 

 attributable to the water itself. 



As regards the light from the sky, the theory which 

 attributes it to dispersal from small particles, many of 

 which are smaller than the wave-length of light, is now 

 pretty generally accepted.. To a first approximation, at 

 any rate, both the polarisation and the colour of the light 

 are easily explained, .\ccording to the simplest theory, the 

 polarisation should be absolute and a maximum at 90° 

 from the sun, and the colour should be modified from that 

 of the sun according to the factor A.-* ; but it is easy to 

 see that there inust be complications, even if all the 

 particles are small and spherical. The light illuminating 

 them is not merely the direct light of the sun, but also 

 light diffused from the sky and from the earth's surface. 

 On these grounds alone the polarisation must be expected 

 to be incomplete even at 90°, and the certain presence of 

 particles not small in comparison with the wave-length is 

 another cause operating in the same direction. It is rather 

 remarkable that, as I noticed in 187 1, the two polarised 

 components show much the same colour. The observation 

 is best made with a double-image prism mounted near one 

 end of a pasteboard tube, through which a suitable 

 rectangular aperture at the other end is seen double, but 

 with the two images in close juxtaposition. When this is 

 directed to a part of the sky 90° from the sun, and the 

 tube turned until one image is at its darkest, the two 

 polarised components are exhibited side by side in a manner 

 favourable for comparison of colours. The addition at 

 the eye end of a Nicol capable of rotation independently 

 of the tube gives the means of equalising the brightnesses 

 without altering the colours. This observation, made in- 

 dependently by Spring, is regarded by him as an objection 

 to the theory, and as showing that the cause of the blue- 

 ness and of the polarisation is not the same. The argu- 

 rnent would have more weight if the colours of the two 

 components were exactly the same and in all circum- 

 stances, but I do not think that this is the case. Observa- 

 tions on the purer sky, to be seen from great elevations, 

 would be of interest. The question is to what causes the 

 • second component is principallv due. So far as it depends 

 •upon sky illumination, it would be bluer than the first 

 component. Any "residual blue" of the kind described 

 bv Tyndall, and due to particles somewhat too big for the 

 simple theory, would make a contribution in the same 



NO. 2106, VOL. 83] 



direction. On the other hand, large particles under the 

 direct light of the sun, and perhaps small ones, so far as 

 illuminated by light from the earth, would contribute a 

 whiter light. In this way an approximate compensation 

 may occur, but the matter is certainly worthy of further 

 attention. 



In this connection it should be noticed that, according 

 to the now generally received electromagnetic theory, com- 

 plete polarisation at 90° requires that the dispersing 

 particles should behave as if spherical, even although 

 infinitely small. If the shape be elongated, there would 

 be incomplete polarisation combined with similarity of 

 colour even under the simplest conditions. 



When the particles are no longer very small in com- 

 parison with the wave-length, the direction of maximum 

 polarisation was found by Tyndall to become oblique, and 

 the deviation is in the opposite direction to that which 

 would have been anticipated from the Brewsterian law for 

 the reflection of light from surfaces of finite area. As I 

 showed in 1881, the gradual precipitation of sulphur from 

 a very weak and acid solution of '* hypo " exhibits the 

 phenomena remarkably well. At a certain stage, depend- 

 ing on the colour of the light, the direction of maximum 

 polarisation becomes oblique. Even when the obliquity is 

 well established for blue light, red light still continues to 

 follow the simpler law, and the comparison gives curious 

 information concerning the rate of growth of the particles. 



The preferential scattering of light of short wave-length 

 involves, of course, a gradual yellowing and ultimate 

 reddening of the light transmitted. The formation in this 

 wav of sunset colours is well illustrated by the acid hypo. 



That Spring rejects this theory in favour of one which 

 would attribute sky-blue to absorption by oxygen or ozone 

 has been already alluded to. .\lthough one must not con- 

 clude too hastily from the behaviour of these bodies when 

 liquefied, it is, of course, possible that their absorbing 

 qualities may influence atmospheric phenomena in some 

 degree ; but to attribute the blue of the sky to them seems 

 out of the question. It is sufficient to remark that the 

 setting sun turns red, and not blue. 



An interesting question remains behind. To what kind 

 of small particles — dispersing short waves in preference — 

 is the heavenly azure due? That small particles of saline 

 or other solid matter, including organic germs, play a 

 part cannot be doubted, and to them may be attributed 

 much of the bluish haze by which the moderately distant 

 landscape is often suffused ; but it seems certain that the 

 very molecules of air themselves are competent to scatter 

 a blue light not ver>- greatly inferior to that which we 

 actually receive. Theory allows a connection to be estab- 

 lished between the transparency of air for light of various 

 wave-lengths, and its known rpfractivity in combination 

 with .Avagadro's constant, expressing the number of 

 molecules per cubic centimetre in gas under standard atmo- 

 spheric conditions. The first estimate of transparency was 

 founded upon Ma.xwell's value of this constant, viz. 

 1-9x10''. Recent researches have shown that this number 

 must be raised to 276x10", and that the result is prob- 

 ablv accurate to within a few per cent.' It has been 

 pointed out bv Dr. Schuster that the introduction of the 

 raised number into the formula almost exactly accounts 

 for the degree of atmospheric transparency observed ^ at 

 high elevations in the United States, apparently justifying 

 to the full the inference that the normal blue of the sky is 

 due to molecular scattering; but, although there is no 

 reason to anticipate that this general conclusion will be 

 upset, it should not be overlooked that a molecule, especi- 

 ally a diatomic molecule, can hardly be supposed to behave 

 as' if it were the dielectric sphere of theor>-. Questions 

 are here suggested for the decision of which the time is 

 perhaps not yet ripe. 



p.S. — The question of the colour of the Mediterranean 

 and other waters was long ago discussed by Mr. J. Aitken 

 — an excellent observer — in Proc. Roy. Soc. Edin., 1881-2. 

 His principal conclusions are very similar to my own. 

 Mr. .'Mtken rightly insists upon the influence of the colour 

 of the suspended matter to which the return of the light 



1 It !k a curious instan'-e of divergence in scientific opinion that whil 

 .some s:ill deny the e xistence of molecules, others have successfully counted 

 them. 



