50 MEMOIR III. 



even before the feeble light of the moon, and also, that it 

 increases with the agitation of the sea, so that during the 

 prevalence of storms it greatly diminishes the dense gloom 

 which at such times is often impenetrable to the moon or 

 stars, throws such a light upon the Ship and rigging as to 

 enable the sailors to execute their allotted tasks with 

 certainty, and at all times points out to the cautious 

 mariner the lurking danger of sunken rocks, shoals, and 

 unknown coasts, by the phosphorescent or snowy appea- 

 rance which it gives to the Breakers, so as to render jthem 

 visible at a considerable distance j where again the diflfused 

 luminous appearance (described under C.) of the Sapphirina 

 indicator is seen, he may be certain that he is in soundings, 

 and probably at no great distance from some fatal spot. 



In the terrestrial animals which are luminous, we perceive 

 organs especially provided to secrete and treasure up the 

 luminous matter, and transparent spots to permit the 

 transmission of the light ; in marine animals nothing of 

 this kind has ever been discovered, and their bodies appear 

 so homogeneous and transparent, that wherever the focus 

 of light may be, when excited, it seems to pervade, and as 

 it were light up the whole body of the animal. Dr. Smith 

 indeed, during the interesting voyage of Captain Tuckey to 

 the Congo, observed that the luminosity of a kind of shrimp 

 appeared to emanate from the brain, which " when the ani- 

 mal was at rest resembled a most brilliant amethyst about 

 the size of a large pin's head, and from which, when it moved, 

 darted flashes of a brilliant silvery light " Tuckey' s Voyage. 

 Spallanzani with his usual ingenuity and perseverance, 

 resorted to a variety of expedients to ascertain where the 

 luminous property resided in the phosporescent medusa 

 of the Mediterranean, and came to the conclusion, that it 

 is confined to the viscid excretion which is found towards 

 the margin of the umbel, on the larger tentacula and " on 

 the surface of the purse communicating with that aperture 

 of the umbella which is perhaps the mouth of the animal. " 



