Ch. XXriI.] SPONTANEOUS EMISSION OF LIGHT. 403 



ous appearances, they must habitually swim at a considerable 

 depth. I never was able to make out any definite outline of 

 the light, which always appeared more or less spherical with 

 faint edges, and sometimes the size and faintness of the 

 flashes seemed to prove that the light must have been dif- 

 fused by its passage through a great depth of water, which 

 would also accoimt for the whitish appearance of what is 

 probably really greenish light. But I am strongly of opiuion 

 that the sources of the flashes and of the moon-shaped 

 patches are identical — in the one case emitting their light 

 spontaneously, and in the other under the excitation of the 

 eddies produced by the ship, and especially by the screw- 

 propeller when at work. 



Before quitting the subject of these flashes I must not 

 omit to mention that while at Singapore, having taken some 

 small Medusae in a towing-net in the straits, I placed them 

 in a glass which stood by my bedside. In the night I ob- 

 served them flashiug brightly with instantaneous flashes, of 

 the same character as those above referred to, although not 

 the slightest shaking was applied to the bottle or irritation 

 to the animals. So also the Noctilucae of Singapore har- 

 bour, which I kept similarly in a bottle, flashed frequently 

 with rapid and bright coruscations ; and I am strongly dis- 

 posed to believe that luminous marine animals, in health, 

 and acting spontaneously without external irritation, always 

 exhibit their luminosity in this manner ; and that it is only 

 when strong excitation is applied that they give out a steady 

 but temporary glow. 



There remains but one form of luminosity to be noticed, 

 which, although I have never been so fortimate as to witness 

 it myself, has been observed by others who have been longer 

 at sea than I was. This is what has been caUed milky sea, an 



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