EFFECTS OF WATER UPON LIGHT 373 



and letting through an excess as compared with that of a freshwater tele- 

 ost? No experiments have been made to test this interesting possibility. 



Effects of Water upon Light — Some other properties of water, salt 

 or fresh, which affect aquatic vision per se regardless of the interspecific 

 variation of fishes in their general make-up, are those which alter the 

 amount and kind of light passing into and through it. To a vastly greater 

 extent than in air, horizontal distances and vertical distances through 

 water are not optically equivalent. As sunlight penetrates downward into 

 water, it undergoes extinction, which is a blanket term embracing both 

 absorption and scattering. Off Plymouth, England, 90% of white light 

 was found to be extinguished at eight or nine meters, 99% at 35 meters. 

 These effects vary from one body of fresh water, or part of the sea, to 

 another and there is no close agreement between investigators as to what 

 is normal or average. Roughly however, a depth of 535 meters in the 

 clearest waters is characterized by utter darkness as far as human vision is 

 concerned. Beebe found only a 'bluish glow' at 435 meters off Nonsuch 

 Island. Some seas are completely dark at 200 meters, dirty harbors in a 

 few meters. Even the clear Bermudian seas seemed to Beebe the 'blackest 

 spot on earth' at a depth of half-a-mile. 



The various wavelengths of light do not all reach to the same depth. 

 The ultra-violet is almost all eliminated in a few millimeters of water, 

 though traces reach to greater depths than any other wavelengths — 

 enough, at 1000 meters, to affect a photographic plate, though only after 

 80 minutes exposure! The infra-red (heat) rays are cut out in a few cent- 

 imeters, or a meter or so. As the light descends, the ends of the spectrum 

 are pared off, the long-wave end more rapidly (90% of the red is gone 

 at five meters) , until a band is left whose wavelengths continue to pene- 

 trate about equally well. The limits of this band, as determined by a 

 photographic technique, are from A,510m[X to X540m[J,. Beebe, in the 

 bathysphere, reported that at 250 meters all that was left of the spectrum 

 was a narrow band centering at X520m[l. This visual observation checks 

 well enough with the results obtained by lowering remote-control cameras 

 to various depths. Not that there is much of this light — 90% of the 

 green part of sunlight is already extinguished at thirteen meters. 



It is more than a coincidence that these best-penetrating wavelengths 

 should be identical with those to which fish rhodopsins are most sensitive. 

 The rhodopsins of land animals have their absorption maxima averaging 

 at around A,500m(J, (that of the ox, for example, is X495m^ — of man, 

 about A,500m|l) ; but it must be remembered that the first rhodopsin was 



