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



was followed by a faint glow which vanished gradually, but whether 

 this was an optical illusion of the retina or not, I cannot be sure. 



Whatever may be the animals which produce these luminous 

 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 diffused by its passage through a 

 great depth of water, which would also account for the whitish 

 appearance of what is probably really greenish light. But I am 

 strongly disposed to believe 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 observed them flashing 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 Noctilucse of 

 Singapore harbour, which I kept similarly in a bottle, flashed fre- 

 quently with rapid and bright coruscations; and I am strongly 

 disposed 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 fortunate as to witness it myself, 

 has been observed by others, who have been longer at sea than I. 

 This is what has been called milky sea, an extraordinary phenomenon 

 of rare occurrence. It has been described to me as a general 

 luminous glow, not confined to the crests of ripples or to disturbed 

 water, but occurring in perfectly calm weather, and looking as 

 though the whole sea was composed of a whitish fluid like milk, 

 with no bright spots or sparks. Such an appearance reflecting 

 a faint light upwards nluminates the ship, rendering every part of 

 the rigging plainly visible, and inasmuch as it can only be seen in 

 the absence of the moon, the contrast of the white glowing sea with 

 the black sky produces an effect calculated to strike the observer 

 with a kind of awe. Although I have met with persons who tell 

 me they have not unfrequently seen this phenomenon, I am disposed 

 to believe that it is extremely rare. One who has not really seen it 

 at all might erroneously suppose, that such an appearance as I 



