1867.] On the Luminosity of the Sea. f)05 



light through a depth of water. I continued watching for a long 

 time in hopes of seeing another, hut although so good an oppor- 

 tunity did not occur again, many seemed to come near the surface, 

 diminishing in size, hut increasing in brilliancy as they did so ; one 

 particularly low down, suddenly gave out a dazzling brilliancy, 

 producing a momentary effulgence all around. 



I may mention that on a moonlight night when the moon has 

 been dimmed by fleecy clouds, I have been able to see the moon- 

 light patches, but when the moon shone out clearly they were no 

 longer visible. 



I have now to describe the fourth form of luminosity exhibited 

 by marine animals, viz. momentary recurrent flashes of light. This 

 form is nearly as commonly seen as the moon-shaped patches 

 already described, which it very frequently, although perhaps not 

 always, accompanies. If, however, the latter are well marked, the 

 flashes are almost sure to be visible. I first observed them in the 

 Indian ocean, north of the line, and since then, in the China seas 

 and Atlantic. This appearance is very striking, but can only be 

 seen under favourable circumstances, i. e. when the night is dark 

 and the sea smooth. An indistinct transitory patch of light appears 

 in the water, as evanescent as a flash of lightning ;• so rapidly does 

 it come and go that it is difficult to fix the exact spot where it 

 occurred. The brightness of the flash varies probably according 

 to the depth of the animal producing it below the surface ; sometimes 

 it is of considerable brilliancy, and sometimes so pale that it would 

 not have been noticed but for its suddenness. The colour is always 

 whitish, and the form of the flash round, brightest in the middle, 

 and becoming indistinct at the circumference. I have on some 

 occasions seen these flashes occur in such numbers and with such 

 rapidity that it would be impossible to count them, though more 

 commonly they are comparatively few and far between. 



But the fact which interested me most in these flashes of light 

 was that they always occurred at a distance from the path of the 

 ship. Although I have seen them accompanying the moon-shaped 

 patches of light in the ship's wake, the places from which I could 

 best observe the flashes were the forecastle or the gangways, when 

 they could be seen in the smooth water several yards distant from 

 the ship's side, and entirely uninterfered with by the ship's motion. 

 This fact proved to me that there were spontaneous emissions of 

 light by some animals below the surface, which voluntarily and at 

 intervals gave out a bright coruscation. Moreover, although 

 rarely, on following with the eye the spot where the flash appeared, 

 it could be seen to reappear further astern, as though the emission 

 was recurrent at definite intervals, as is the case with the luminous 

 beetles called fire-flies at Singapore. I have also noticed on more 

 than one occasion that the flash, instead of instantly disappearing, 



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