248 



DEPTHS OF THE OCEAN 



The penetra- 

 tion of light 

 into the sea. 



Absorption of 

 light rays. 



Intensity of 

 light at 

 different 

 depths. 



Fol and 

 Sarasin. 



When the sun's rays fall on the surface of the sea, some of 

 them are rejected, and the rest penetrate into the water, though 

 in a somewhat altered direction. The direction is not much 

 altered when the sun is high in the heavens, as at noon in the 

 tropics. When the sun is just above the horizon its rays are 

 most strongly deflected, the few rays penetrating into the water 

 forming an angle of about 42" with the surface. As the sun 

 rises and the light becomes more intense, the deflection from 

 the course in the air gradually decreases, so that the rays do 

 not penetrate so deep as might be expected, even if the 

 angle with the surface increases. When the sun is 60^ above 

 the horizon, the refraction in the water is about 8°, the angle 

 between the surface and the penetrating rays then being about 

 68°, and when the sun is at its zenith, the rays are not bent at 

 all, but proceed perpendicularly into the water. 



The rays making their way into the water are, however, 

 gradually absorbed, some quickly, others more slowly, accord- 

 ing to the wave-length of the ray and the limpidity of the water. 

 The sun's light, of course, consists of many different kinds of 

 rays : the dark heat-rays, imperceptible to the eye, lie beyond 

 the red end of the spectrum, and are therefore called ultra-red 

 rays ; then comes the visible spectrum with the colours in the 

 well-known order — red, orange, yellow, green, blue, indigo, and 

 violet ; beyond the violet end are the ultra-violet rays, remark- 

 able for their chemical action, but having no effect on our 

 senses. These different rays are refracted and absorbed in 

 different degrees. The red rays are refracted somewhat less 

 than the blue and violet rays, and are much more quickly 

 absorbed. The dark heat-rays are absorbed in the very upper- 

 most water-layers. The light rays also convey some heat, and 

 they penetrate deeper before disappearing — the deeper the 

 nearer the blue end of the spectrum is approached. Light at a 

 certain depth in the sea has not the same composition as on 

 the surface of the earth, there being fewer of the red rays 

 and more of the blue, which proportion becomes gradually more 

 pronounced with increasing depth. 



Attempts have been made to determine the intensity of the 

 light at different depths, especially in the Mediterranean, by 

 means of the action of the rays on photographic plates. 

 Ordinary plates are most influenced by the rays at the blue end 

 of the spectrum, and by the ultra-violet rays, and only slightly 

 by the red. Fol and Sarasin, working off the Riviera, traced 

 an effect on the plate as far down as between 465 and 480 



