9*f% Mr. Macartney’s Observation? 
sudden and general effort of the medusa* to separate from 
each other, and descend in the water. 
The medusa scintillans almost constantly exists in the 
different branches of Milford haven that are called pills. I 
have sometimes found these animals collected in such vast 
numbers in those situations, that they bore a considerable 
proportion to the volume of the water in which they were 
contained : thus, from a gallon of sea water in a luminous 
state, I have strained above a pint of these medusae. I have 
found the sea under such circumstances to yield me more 
support in swimming, and the water to taste more disagreeably 
than usual ; probably the difference of density, that has been 
remarked at different times in the water of the sea, may be 
referred to this cause. 
All my own observations lead me to conclude, that the 
medusa scintillans, is the most frequent source of the light 
of the sea around this country, and by comparing the accounts 
of others with each other, and with what 1 have myself seen, 
1 am persuaded that it is so likewise in other parts of the world. 
Many observers appear to have mistaken this species for the 
nereis noctiluca, which was very natural, as they were pre- 
possessed with the idea of the frequent existence of the one, 
and had no knowledge of the other. Some navigators have 
actually described this species of medusa, without being aware 
of its nature. Mr. Bajon, during his voyage from France to 
Cayenne, collected many luminous points in the sea, which 
he says, when examined by a lens, were found to be minute 
spheres. They disappeared in the air. Doctor Le Roy, in sail- 
ing from Naples to France, observed the sparkling appearance 
of the sea, which is usually produced by the medusa seintillansv 
