July 21, 19 10] 



NATURE 



87 



COLOUR OF THE SEA. 



■ fPROPOS of the report (Nature, March lo) of Lord 

 ■^ Rayleigh's lecture deahng with the parts played by 

 reflection and transmission of light in the production of 

 the integral impression of colour on the eye of an observer 

 looking at the sea from the deck of a ship, I should like 

 to be permitted to make some observations on the proper 

 colour of the water of the ocean, as it is a subject which 

 has occupied my attention, off and on, during the last 

 fortv years. 



During the voyage of the Challenger I began to log the 

 colour of the water in February, 1S74, when she was 

 working in the neighbourhood of the Antarctic circle. My 

 attention was there directed to it by the frequent and 

 abrupt passage of the ship from water of the clear indigo 

 colour of the ocean of temperate latitudes to the deep 

 olive-green water which is a distinctive feature of these 

 icy regions. 



The colour is due to the abundance of diatoms. These 

 are. so plentiful and so preponderant that, besides putting 

 their stamp on the surface, they furnish a distinct type of 

 oceanic deposit, the diatom-ooze. The green colour of 

 the water is due, not only to the living diatoms, but also, 

 and perhaps to a greater extent, to the excretions of the 

 animals for the subsistence of which the diatoms furnish 

 the ultimate food supply. The crowds of penguins and 

 other birds to be met with in these seas stain all the ice 

 green where they have rested. The water, inhabited by 

 diatoms and affected by diatomaceous debris, has a deep 

 olive-green colour which is characteristic, and this I 

 accepted as one colour-type of the water of the ocean. 

 It is seen best in the water the transparency of which is 

 not interfered with by too great a crowd of the diatoms 

 themselves. Water belonging to this type of colour is not 

 confined to polar latitudes ; it is met with in a certain 

 class of homologous districts of the warmer ocean, in 

 tropical and even in equatorial latitudes. 



When we quit the edge of the polar ice and steer equator- 

 wards, the surface water assumes a pronounced indigo 

 colour, and this persists until we pass the fortieth parallel. 

 If we start from the equator and sail polewards, the 

 colour of the surface water persists as a pure and brilliant 

 ultramarine until the thirtieth parallel is passed. The 

 passage from the ultramarine to the indigo, and -jice 

 versa, is usually very rapid, and the area of mix- 

 ture is restricted. No one who has once sailed in the 

 ultramarine waters of the intratropical ocean and has 

 observed, as well as seen, its colour, can ever mistake 

 any other colour for it. If he has doubt as to whether 

 the water through which he is passing is ultramarine or 

 not, he may be sure that it is not. The ultramarine and 

 the indigo are the two great colour-types to which the 

 mass of the surface water of the deep sea belongs, and, 

 with the olive-green, they make the three fundamental 

 colour-types which are required, and are sufficient for the 

 adequate logging of the colour of the surface water of 

 the ocean. 



The water of the Mediterranean belongs to the ultra- 

 marine type, but it always appears to me to have a 

 harder tone than the soft and brilliant ultramarine of the 

 intratropical ocean. 



With regard to the method of judging the colour of the 

 water, much unnecessary difficulty is made. The first 

 precaution to be observed is to take up a position where 

 the greatest amount of light can reach the eye after pass- 

 ing through the water, and the smallest amount after being 

 reflected from its surface. There is generally little 

 diflficulty in accomplishing this on one side or the other of 

 the ship and by looking as nearly as possible vertically into 

 the water. 



The Challenger, like other men-of-war of her date, was 

 fully rigged, and built for sailing as well as for steaming. 

 \\ hen under sail the propeller causes a certain amount of 

 retardation, and to remedy this she was fitted with a 

 " screw-well " into which the propeller could be hoisted 

 out of the water. This proved to be a perfect observation 

 tube for determining Jhe proper colour of the water. Its 

 diameter was about 6 feet : it passed from the upper deck 

 through the captain's cabin on the main deck and the 

 ward-room on the lower deck into the water. Looked 

 into from the deck, the sea water appeared to be enclosed 



NO. 2125, VOL. 84] 



in it as the water is in a well, but with this difference, 

 that the water, by day, was brilliantly illuminated from 

 below. There being no clearance between the surface of 

 the water in the well and the structure of the ship, no 

 light could enter except through the water. No direct 

 sky-light could reach it down the well, because the poop 

 awning, which was practically always spread during the 

 dav, completely excluded it. The screw-well was, in 

 effect, an artificial and perfected Grotto di Capri, which 

 was carried round the world. It was perfected, inasmuch 

 as there is a passage for boats to penetrate into the grotto 

 from the outside, while the screw-well is entirely shut off. 

 During the whole of the voyage th'e colour ot the water 

 was under observation in this very perfect apparatus. 



The statement that the blue colour of the sea is nothing 

 but the reflection of the blue of the sky was at first fre- 

 quently made, even on days when the sky was completely 

 overcast ; a visit to the screw-well, especially on overcast 

 days, never failed to convince the doubter that the water 

 contained in its own mass sufficient colour to account for 

 all that was perceived. When the ship was in green water 

 the view was never advanced that its colour was due to 

 reflection from the sky. 



.'\s ships with screw-wells long ago disappeared from 

 the sea, it may not be superfluous to point out that 

 what could be observed in the screw-well was altogether 

 different from what can be seen in the wake of the screw 

 of a modern steamer. While the screw-well was a per- 

 fect instrument for gauging the colour of the water, the 

 determination of its transparency was more conveniently 

 made from a boat. Thus in mid-Pacific, with the aid of 

 a " water-glass " to eliminate the disturbing action of 

 ripples, a metal plate measuring only 4 by 4 inches, 

 painted white and not masked by the suspending line, 

 was distinctly seen at a depth of 25 fathoms (45 metres). 

 Beyond this depth it became indistinct, and became 

 invisible at about 27 fathoms, but this was due mainly to 

 its smallness and to its want of steadiness, being attached 

 to the boat, which rose and fell with the swell. .■\t 25 

 fathoms the plate had a pale ultramarine colour, and its 

 edges were sharply defined. These separated the column 

 of water, into which I looked through the water-glass, into 

 a central column of rectangular section having a depth of 

 25 fathoms, and into a column, surrounding and contiguous 

 with it, which had a depth many times greater. These 

 columns, being juxtaposed, were placed in the way most 

 favourable for the comparison of their colours. The colour 

 of the central column, 25 fathoms in length, was a pure but 

 pale uhramarine ; that of the external and uninterrupted 

 column through which the whole unabsorbed and _un- 

 dissipated part of the sunlight which had penetrated into 

 the water returned to the surface was of the same torie, 

 but of many times greater intensity, .'\ssuming the in- 

 tensity of the colour to be proportional to the length of 

 the column of water traversed by the light, it is to 

 be concluded that the length of the uninterrupted column 

 which transmitted the more intense colour was many times 

 greater than 25 fathoms. It must be noted that the glass 

 plate forming the bottom of the small tub, which is called 

 a " water-glass," was during the observation completely 

 protected from direct sky-light by my head and the brim 

 of the panama hat which, at that time, I always wore 

 when exposed to the sun. 



It has already been said that water of as pure a green 

 as that of the Antarctic occurs in other and warmer dis- 

 tricts of the ocean. My attention was first directed to 

 this during the cruise of the Dacia, which, akhough it 

 occupied no more than three weeks, marks an_ epoch in 

 deep-sea research. A short account of it is given in a 

 paper by me — " On Oceanic Shoals discovered in SS. Dacia 

 in October, 188^ "—and published in the Proceedings of the 

 Royal Societv of Edinburgh, 1S85, xiii., p. 74S. Perhaps 

 the most remarkable of these shoals was the one which 

 was named the " Coral Patch," in lat. 34° 57' N., long. 

 11° 57' W., the exploration of which, along with that of 

 the tidal currents in the open ocean (Proc. Roy. Soc, 1SS8, 

 xliii., p. 3^6), supplied the evidence which definitively estab- 

 lished the 'fact that coral islands are a product of elevation 

 and not of subsidence. 



When the survey of this shoal had been completed, in 

 so far as the time at the disposal of a steamer engaged on 

 a commercial mission permitted, a line of soundings was 



