Ch. XXIII.], CAUSE OP MOON-SHAPED PATCHES. 399 



water at different depths. Sometimes, when deep down, 

 they were pale, and of a whitish colour, with indistinct out- 

 line, and of large size ; but when nearer the surface, they 

 , w^e smaller and more distinct, and assumed a pale greenish 

 tinge. They usually remained visible for eight or ten se- 

 conds, but sometimes less. As these appearances were just 

 such as might be presented by the umbrellas of large Me- 

 dusae, were such present and luminous, I was strongly in- 

 clined, at first, to attribute them to this cause ; and the 

 fact that on one occasion (about a week after I left Eng- , 

 land), I saw these moonlight patches in the Red Sea on 

 the evening of a day on which the ship had passed through 

 a shoal of Aurelise, led me to attribute them to their pre- 

 sence. I supposed that the Aureliae, struck by the screw, 

 gave out their light under the excitation of the blow, and 

 floated away luminous and dying. But I was forced to 

 abandon this theory afterwards ; for I have since many times 

 watched for floating- Medusae before the light failed, aiid 

 have not seen 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 luminous appearances 

 have not been more marked than usual, but even less so. 

 Moreover, having secured specimens of these Acalephs, 

 they have not exhibited any luminosity whatever during 

 the night. Although, however, I ceased to regard the 

 Acalephse as the source of the luminous patches in 

 question, there can be no doubt that the gi-eat nimibers 

 which are always visible immediately under the stem, are 

 due to the fact of the eddies of the ship exciting the emis- 

 sion of light in certain animals capable of exhibiting lumi- 

 nosity. Not, however, that similar appearances are never 



