1867.] On the Luminosity of the Sea. 507 



have already alluded to as having twice occurred to me on the 

 coast of China (when the ship seemed to he sailing in a luminous 

 sheath) corresponded to the description of a milky sea, and in a 

 small way it did so, and I considered it, at the time, as the nearest 

 approach to it I had ever observed. But the milky sea must be 

 something sui generis, and I imagine it to be owing rather to a 

 condition of the water under certain peculiar atmospheric or 

 climatic influences than to any extraordinary number of luminous 

 animals in the water. A circumstance which occurred to me 

 seemed to throw some light upon the subject and confirmed me in 

 this opinion. Having put down the towing-net in the Formosa 

 Channel it collected a number of small entomostraca, megalopas, 

 minute medusae, small porpitae, pteropods, annelids, globigerinae, 

 &c, which I placed in a basin of sea water, and not having finished 

 my examination of them they remained upon the table during the 

 night. On stirring the water in the dark the whole became faintly 

 luminous, giving out a general glow as if every particle were 

 phosphorescent, the minute Crustacea, &c, appearing as bright spots 

 in the luminous fluid. If the slimy substance, in which in some 

 marine animals at least the luminous property appears to reside, 

 become diffused through the water, as it is probable it may be 

 under certain combinations of conditions and circumstances, a 

 general luminosity of the water may result, similar to that observed 

 in milky sea, while its small sparks, doubtless in great abundance, 

 would remain unnoticed in the universal glow, but would at the 

 same time greatly enhance the general luminous effect. 



There is a common idea that a southerly wind is peculiarly 

 productive of luminosity in the sea, but according to my observa- 

 tions this is an error. The winds most prevalent when luminosity 

 has been well marked have been westerly, north-westerly, or even 

 easterly, south being perhaps the least frequent ; but probably the 

 direction of the wind has no special influence in the matter. 

 What the favourable conditions really are it is as difficult to say 

 as it is in the case of floating animals generally. I have seen 

 remarkable exhibitions on one night followed by nearly absolute 

 darkness on the next, the conditions of wind, weather, barometer 

 and thermometer, being inappreciably altered. Probably tempera- 

 ture is as important as any influence ; the luminosity in the Mersey 

 only occurs in summer, and in rounding the Cape of Good Hope 

 during the winter season, scarcely any luminosity was exhibited 

 during the month tnat we were passing through the higher degrees 

 of S. latitute. 



The animals which I have observed to possess luminous proper- 

 ties are not numerous. Many of the more minute animals taken 

 in the towing-net appear to exhibit them, more particularly the 

 small Crustacea (Entomostraca) and small Medusae (Medusidae). 



