EFFECTS OF WATER UPON LIGHT 375 



from our immensely remote aquatic ancestors. If rhodopsin had been first 

 invented on land, it might very well have been purple, not red. 



In some freshwater and anadromous fishes it has recently been found 

 that the absorption maximum is roughly intermediate between those of 

 marine fishes and land animals, the value being ^522 ± 2m|i,. The investi- 

 gator (Wald) has called the photosensitive substance involved 'porphy- 

 ropsin'; but there is little excuse for the new word. Rhoddpsins have been 

 re-invented so many times that if we coined a new name for each one 

 that we can distinguish chemically or spectroscopically, the nomenclature 

 would soon be hopelessly confused. There is even good reason to think 

 that in marine fishes there are two rhodopsins simultaneously present, 

 the effective absorption maximum of the rods being dependent upon the 

 relative amounts of the two. 



An important effect of depth upon light-quality is happily significant 

 for every fish. This is the rapid extinction of ultra-violet. In this part of 

 the spectrum there is one band of wavelengths, from 295m[i to 305m[l, 

 which is particularly harmful — positively lethal, in fact — to living tissues. 

 This is consequently known as the 'abiotic' range. No aquatic species need 

 concern itself with protection from abiotic light; for even if the water 

 were chemically pure, a few millimeters would absorb it all. The dis- 

 solved and suspended matter of natural water disposes of it even more 

 promptly, by fluorescing it into harmless visible light. Land vertebrates, 

 diurnal ones at any rate, have had to evolve a capacity for fluorescence 

 by their lenses. Aquatic forms frequently show no such capacity, for they 

 do not need it. Stickleback and toad lenses are very transparent to ultra- 

 violet, frog and carp lenses less so. 



More important than the qualitative effects of water upon light are its 

 quantitative ones. Even if extinction with depth were not selective, it 

 would still affect aquatic vision profoundly — as is emphasized by the vis- 

 ual problems of the deep-sea fish, shortly to be considered. Even close to 

 the surface, vision in the horizontal direction is greatly dimmed. To put 

 the matter crudely, we cannot see the side of a light-beam — only its end, 

 and for light to enter the eye it must be sent or reflected directly toward 

 the organ. Under water, there are not nearly as many reflecting objects 

 in the plane of the eye of a fish — particularly in the open sea — as there 

 would be on land. Such light as is aimed at the eye is weaker because 

 there are few hard and smooth, hence brightly reflective, surfaces; and 

 this weak light is further weakened through scattering by suspended 

 matter. All in all, if a fish or whale can distinguish objects fifty feet 



