48 



NATURE 



[March io, 1910 



COLOURS OF SEA AND SKY.' 



A RECENT voyage round Africa recalled my attention 

 to interesting problems connected with the colour of 

 the sea. They are not always easy of solution in conse- 

 quence of the' circumstance that there are several possible 

 sources of colour the action of which would be much in 

 the same direction. We must bear in mind that the 

 absorption, or proper, colour of water cannot manifest 

 itself unless the light traverse a suf!icient thickness before 

 reaching the eye. In the ocean the depth is, of course, 

 adequate to develop the colour, but if. the water is clear 

 there is often nothing to send the light back to the 

 observer. In these circumstances the proper colour cannot 

 be seen. The much admired dark blue of the deep sea 

 has nothing to do with the colour of water, but is simply 

 the blue of the sky seen by reflection. When the heavens 

 are overcast the water looks grey and leaden ; and even 

 when the clouding is partial, the sea appears grey under 

 the clouds, . though else.where.it may show .colour. It is 

 remarkable that a fact so easy of observation is unknown 

 tb many. even of those who have written from a scientific 

 point of view. One circumstance which may raise doubts 

 is that the blue of the deep sea often looks purer and 

 fuller than that of the sky. I think the explanation is 

 that we are apt to make comparison with that part of the 

 sky which lies near the horizon, whereas the best blue 

 comes from near the zenith. In fact, when the water is 

 smooth arid the . angle of observation such as to reflect the 

 low sky, the apparent blue of the water is much 

 deteriorated. In these circumstances a rippling due to 

 \*ind greatly enhances- the colour by reflecting light from 

 higher up. Seen from the deck of a steamer, those parts 

 erf the Waves which slope towards the observer show the 

 best colour for a like reason. 



The real colour of ocean water may often be seen when 

 there are breakers. Light, perhaps directly from the sun, 

 may then traverse the crest of the waves and afterwards 

 reach the observer. In my experience such light shows 

 decidedlv green. Again, over the screw of the ship a good 

 deal of "air is entangled and carried down, thus providing 

 the necessary reflection from under the surface. Here also 

 the colour is green. 



The only places where I have seen the sea look blue 

 jn a manner. -not explicable by reflection of the sky were 

 Aden and Suez. Although the sky was not absolutely 

 overcast, it seemed that part, at any rate, of the copious 

 if riot very deep blue was to be attributed to the water. 

 This requires, not only that the proper colour of the water 

 should here be blue, ' but also the presence of suspended 

 matter capable of returning the Hght, unless, indeed, the 

 sea bottom itself could serVe the purpose. 



The famous grotto at Capri gives an unusijally good 

 opportunity of seeing the true colour of the water. Doubt- 

 less 'a great part of the effect is- due- 16 the eye being 

 shielded from external glare, and so better capable of 

 appreciating the comparatively feeble light which has 

 traversed considerable thickriesses of water. The question 

 was ■ successfully discussed many years ago by Melloni, 

 who remarks that the beauty of- the colour varies a good 

 de.-il with the weather. The light which can penetrate 

 comes from the sky, and ndt directly from the sun. When 

 the day is -clear the blueness of the sky cooperates with 

 th" blueness of the water. . ■ 

 . That light- reflected from the surface of a" liquid does 

 not exhibit the absorption colour is -exemplified by brown 

 peaty water such as is often met with in Scotland. The 

 skv seen by reflection is as blue as if the water were pure ; 

 but an attempt to illustrate this fact by experiment upon 

 quite a small scale was not at first successful. A large 

 white photographic dish containing dark-brown oxidised 

 " pyro " was exposed upon the lawn during a fine day. 

 Although the reflected light certainly came from the clear 

 skv, the colour did not appear pronounced, partly in con- 

 5-.'-quence of the glare of the sunshine from the edges of 

 the dish. The substitution of a dish of glass effected an 

 improvement; but it was only when the eye was protected 

 fro-^n extraneous light by the hands, or more perfectly by 

 the interposition of a pasteboard tube held close up, that' 

 the blue of the reflected light manifested its proper purity. 

 1 D^snourte delivered at the Royal Institution on Friday, February 25, by 

 the Right Hon. Lord Rayleigh, O.M., F.R.S. 



It would seem that the explanation is to be sought in 

 diffusion of light within the lens of the eye, in consequence 

 of which, especially in elderly persons, the whole field is 

 liable to be suffused with any strong light finding access. 



As regards the proper colour of pure water, an early 

 opinion is that of Davy, who, in his " Salmonia," pro- 

 nounces in favour of blue, basing his conclusion upon 

 observations of snow and glacier streams. The latter, 

 indeed, are often turbid, but deposit the ground-up rock 

 which they contain when opportunity offers, as in the Lake 

 of Geneva. A like conclusion was later put forward by 

 Bunsen on the basis of laboratory observations. The most 

 elaborate experiments are those of Spring, who, in a series 

 of papers published during many years, discusses the 

 difficult questions involved. He tried columns of great 

 length — up to 26 metres ; but even when the distance 

 traversed was only 4 or 5 metres, he finds the colour a 

 fine blue only to be compared with the purest sky-blue 

 as seen from a great elevation ; but when the tubes contain 

 ordinary water, even ordinary distilled water, the colour 

 is green or yellow-green, and not blue. 



The conversion of the original blue into green is, of 

 course, explicable if there be the slightest contamination 

 with colouring matter of a yellow character — i.e. strongly 

 absorbent of blue light. Spring shows that this is the 

 effect of minute traces— down to one ten-millionth part — 

 of iron in the ferric state or of humus. The greenness 

 of many natural waters is thus easily understood. Another 

 question exarnined by Spring is not without bearing upon 

 our present subject, viz. the presence of suspended matter. 

 I am the better able to appreciate the work of Spring, 

 that many years ago I tried a variety of methods, including 

 distillation in vacuo, in order to obtain water in the con- 

 dition which Tyndall described as "optically empty," but 

 I met with no success. Spring has shown that the desired 

 result may be obtained by the formation within the body 

 of the liquid of a gelatinous precipitate of alumina or 

 oxide of iron, by which the fine particles of suspended 

 matter are ultimately carried down. 



Perhaps the most telling observations upon the colour of 

 water are those of Count Aufsess, who measured the actual 

 transmission of light belonging to various parts of the 

 spectrum. The principal absorption is in the red and 

 yellow. In the case of the purest water, there was practic- 

 ally no absorption above the line F, and a high degree of 

 transparency in this region was attained even by some 

 natural waters. That these waters should show blue, when 

 in sufficient thickness, is a necessary consequence. 



In my own experiments, made before I was acquainted 

 with the work of Aufsess, the light traversed two glass 

 tubes of an aggregate length of about 4 metres (12 feet). 

 On occasion the light was reflected back so as to traverse 

 this length twice over. I must confess that I Tiave never 

 seen a blue answering to Spring's description when the 

 original light was white. For final tests I was always 

 careful to employ the light of a completely overcast day, 

 which was reflected into the tubes by a small mirror. 

 The colour, after transmission, showed itself very sensitive 

 to the character of the original source. The palest clear 

 sky of an English winter's day gave a greatly enhanced 

 blue, while, on the other hand, isolated clouds are usually 

 yellowish, and influence the result in the opposite direction. 

 I should myself describe the best colour of the transmitted 

 light on standard days as a greenish-blue, but there is 

 some variation in the use of words, and, perhaps, in 

 vision. Some of my friends, but not the majority, spoke 

 of blue simply, but all were agreed that the blueness of a 

 good sky was not approached. The waters tried have been 

 very various. Sea-water from outside the grotto of Capri, 

 from Suez, and from near the Seven Stones Lightship, off 

 the Cornish coast, I owe to the kindness of friends. Of 

 these, the two former showed a greenish-blue, the latter 

 a full, or, perhaps, rather yellow-gr.een, and these colours 

 were not appreciably modified after the water had stood 

 in the tubes for weeks. It is important to remember that 

 the hue mav, to some extent, depend upon thickness. It 

 is quite probable that in a greatly increased thickness the 

 Capri and Suez waters would assume a more decided blue 

 colour; but I do not think the Seven Stones water could 

 so behave, the colour, with 12 feet, seeming to involve the 

 absorption of blue light. . • . . 



NO. 2106, VOL. 83] 



