504. On the Luminosity of the Sea. [Oct., 



one for days and weeks together, and yet the moon-shaped patches 

 have been as bright and as abundant as before ; and again, when 

 we have passed through a thick shoal of Medusae towards evening, 

 the lunrinons appearances hare not been more marked than nsual, but 

 even less so. Sloreover, haying seemed one of these Acalephs, it 

 has not exhibited any luminosity during the night. 



Although, however, I ceased to regard the Aealephae as the 

 source of the luminous appearances in question, there can be no 

 doubt that the great numbers which are always visible immediately 

 under the stern are due to the fact of the eddies of the ship exciting 

 the emission of light in certain animals capable of exhibiting 

 luminosity. Not however that similar appearances are never seen 

 in other situations where they are unmolested, though I must say 

 that in my experience this is rare. Thus in the Indian ocean, in 

 lat. 12^° N. and long. 55° E. (bar. 30°, them. 82°), among other 

 appearances I noticed now and then a large patch of light with a 

 roundish irregular outline pass by. emitting a pale and steady light, 

 although out of the path of the ship : and on August 17th, being 

 in a small boat on the coast of Borneo in a strong breeze after 

 dark, I observed deep beneath the surface and entirely apart from 

 any influence of the oars, the appearance of large globes of white 

 light, shining persistently and spontaneously. 



Although I long and constantly watched for the bodies which 

 produced this remarkable and frequent luminous effect, I did so for 

 a long time in vain. In vain I attempted to penetrate below the 

 surface in search of any animals which could possibly ori.sinate the 

 light. Although I could distinctly see the bottom of the ah 

 rudder, 19 feet deep, I could never detect a trace of any living 

 thing within that depth by day, but no sooner did darkness super- 

 vene than they were often in abundance. It was only by accident, 

 on June the 2nd, in lat. 28^° S., and long. 9 : E., that I was 

 witness of a circumstance which seemed to elucidate the question. 

 Looking as usual over the stern, there were plenty of moon-shaped 

 patches, accompanied by sparks unusually large and bright. The 

 patches were remarkably persistent, and could be traced for nearly 

 half a minute after the ship had passed. They were evidently a 

 considerable but varying distance below the surface of the water ; 

 when far down they appeared large and faint and ill defined, but 

 when nearer the surface they were smaller, brighter, and better 

 defined. As I watched, one of the bright bodies whirled about by 

 the eddy of the rudder came absolutely to the surface 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 at once occurred to me that it was a Pyrosorua, and 

 that this Ascidian was the usual cause of the phenomenon, the 

 circular fomi of the patches being produced by the diffusion of the 



