1910] Cnlourfi of Sea and Sky. 765 



WEEKLY EVENING MEETING, 



Friday, February 25, 1910. 



Sir William Crookes, LL.D. D.Sc. F.R.S., Honorary Secretary 

 and Vice-President, in the Chair. 



The Right Hon. Lord Rayleigh, O.M. D.C.L. LL.D. D.Sc. 



F.R.S. 31.RJ. 

 Honorary Professor of Natural Philosophy, Royal Institution. 



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 consequence of the circumstance that there are 

 several possible sources of colour whose action 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 sufficient thickness before reaching the eye. In tlie 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 tlie observer. 

 Under these circumstances the proper colour cannot be seen. The 

 much admired dark blue of the deep sea lias 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 elsewhere it may show colour. It is remarkable that 

 a fact so easy of observation is unknown to 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 tlie explanation is 

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

 Ues near the horizon, whereas the best blue comes from near the 

 zenith. In fact, when the water is smooth and the angle of observa- 

 tion such as to reflect the low sky, the apparent blue of the water is 

 much deteriorated. Under these circumstances a rippling due to 

 wind greatly enhances the colour by reflecting light from higher up. 

 Seen from the deck of a steamer, those parts of 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 hght shows decidedly green. Again, over the screw 



