The Sea-water and its Physical and Chemical Properties 61 



Hemisphere the colder water of the Falkland current and the oceans areas around 

 Bouvet Island are mostly greenish blue to green. 



An explanation of the colour of pure sea-water must be sought, in the first place, 

 in the optical properties of sea-water. The Bunsen theory ascribed the blue colour of 

 the sea to the combined effects of the spectral absorption of pure sea-water and re- 

 flection by the particles suspended in the water (absorption theory). The light entering 

 the water (direct sunlight and diffuse radiation from the sky) will be weakened least 

 in the blue by absorption. Down into the deeper layers the light becomes more and 

 more blue. This relative concentration of blue is further increased in the light reflected 

 from small particles and passing back to the surface, the light returning through the 

 surface is thus blue. Against this absorption theory, Soret has set a diffraction theory 

 according to which the explanation of the blue colour of the sea is analogous to that 

 of the blue colour of the sky and is due to the scattering of light in the water. Ramana- 

 THAN (1923) has attempted to prove by experiment and theoretical investigation that 

 pure sea-water should show an indigo blue colour by molecular dispersion and by 

 selective absorption, and that small amounts of suspended matter have little effect on 

 the colour. According to the theoretical investigations of Gans (1924), the colour is 

 due principally to diffraction of higher orders (see also Lauscher, 1947). 



A third possible explanation for the widely occurring greenish colour was advanced 

 by WiTTSTEiN (1860) and later by Spring (1886, 1898) in the so-called "solution 

 theory". In this, blue was regarded as the actual colour of the water and all variations 

 were due to different substances dissolved in the water. This effect was ascribed prin- 

 cipally to organic humus materials that in increasing concentration made the water 

 first green, then yellowish green and finally, in extreme cases, brown. 



It was first pointed out by Kalle (1938, 1939) that the physiology of colour vision 

 must play a large part in the explanation of the colour assumed by the sea and must be 

 taken into consideration. According to the Young-Helmholtz theory of colour vision, 

 the human eye has three groups of colour-sensitive elements (cones), each of which is 

 sensitive to one of the three primary colours, red, blue and green. The stimulation of 

 two or all three of these groups at the same time gives the impression of a mixed 

 colour. Every different colour impression is produced by a definite ratio in the strength 

 of the stimulation of the three different types of cones. A "colour triangle" (Fig. 34) 

 can be used to represent diagrammatically all possible colour impressions. The three 

 corners of the triangle represent the total (100%) stimulation of only one group of 

 receptors — red, green or blue. At every point on the triangle the sum of the oblique 

 co-ordinates of the point is always 100%, and these co-ordinates represent the per- 

 centage composition of the mixed colour characterized by that point. The point 

 W = white which, by definition, is composed of a mixture of 33J% of each of the three 

 primary colours hes at the centre of gravity of the triangle. All tones of the same colour 

 lie along a straight fine that runs radially from the white point; the nearer a point on 

 such a line lies to one of the sides of the triangle the more saturated is the colour it 

 represents. The position of the spectral colours within the triangle is shown by the 

 curve marked on the diagram. Since the spectral colours are the most saturated 

 colours possible in nature, all colours found in nature must lie on the area within the 

 spectral curve and the line joining its two end-points. 



In the light of the consequences of this theory, Kalle has investigated the effects 



