Cii. XXni.] FLASHES OF LIGHT. 401 



whirled about by the eddy of the rudder, came absolutely to 

 the sui-face, and exhibited a nearly rectangular form of great 

 brilliancy, of a pale green colour, and, as far as I could judge, 

 about six inches long by two broad. It instantly occurred 

 to me that it was a Pyrosoma, and that this Ascidian was the 

 usual cause of the phenomenon, the circular form of the 

 patches beiog only an illusion produced by the diffusion of 

 the light through a certain depth of water. I continued 

 watching for a long time in the hope of seeing another ; but 

 although so good an opportunity did not occur again, many 

 seemed to come near the surface, diminishing in size, but 

 increasing in briUiancy as they did so — one in particular, 

 very low down, suddenly gave out a dazzling brilliancy, which 

 produced a momentary effulgence in the water all around. 



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

 has been dimmed by fleecy clouds, I have been able dis- 

 tinctly to recognise these moonlight patches ; but when the 

 moon has shone out clearly they were no longer visible. 



I have now to describe the fourth form of limiinosity ex- 

 hibited by marine animals, viz., momentary recurrent flashes 

 of Hght. 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 



