7CjQ The Right Hon. Lord RayUigh [Feb. 25, 



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 in 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 not 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 light, unless indeed the sea bottom itself could serve 

 the purpose. 



The famous grotto at Capri gives an unusually good opportunity 

 of seeing the true colour of the water. Doubtless a great part of the 

 effect is due to the eye being shielded from external glare and so 

 better capable of appreciating the comparatively feeble light whicli 

 has traversed considerable thicknesses of water. The question was 

 successfully discussed many years ago by ^lelloni, who remarks that 

 the beauty of the colour varies a good deal with the weather. The 

 light which can penetrate comes from the sky and not directly from 

 the sun. When the day is clear, the blueness of the sky co-operates 

 with the 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 sky 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 oxidized 

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

 reflected light certainly came from the clear sky, the colour did not 

 appear pronounced, partly in consequence of the glare of the sun- 

 shine from the edges of the dish. The substitution of a dish of glass 

 effected an improvement. But it was only when the eye was pro- 

 tected from 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. 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," pronounces 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, 



