258 Remarks on Marine 



power cannot be applied. It is scarcely possible indeed to 

 make effective use of one greater than that produced by a sim- 

 ple lens of half an inch focal distance ; and as, with this 

 power the field of view is very contracted, it is often convenient 

 to have two other lenses at hand of one inch and of two inches 

 in focal distance. The very minute ones may be occasionally 

 secured in a single drop of water under a compound micro- 

 scope ; but the observer will be disappointed much oftener than 

 he will succeed in his attempts to examine them in this way ; 

 partly from the chance of his failing to find any in many suc- 

 cessive small portions of water thus separated, and partly for 

 the reasons just stated. 



I have already mentioned almost all that occurs on the me- 

 thod used in determining those species which were luminous. 

 Of the larger kinds, it seldom happened that more than two or 

 three, sometimes not more than one, was contained in a tumbler. 

 Being placed in the dark, and stirred with the finger, the same 

 number of sparks were produced ; and whatever failure might 

 here have occurred in one trial, was removed by others made 

 at different times. With regard to the smaller species, it some- 

 times happened that only one was found on a particular 

 occasion, and the luminous state of that water on agitation 

 proved the property to exist in that individual species. Re- 

 specting some of these species, however, doubts may remain ; 

 as in some cases no one of them was found alone. But these 

 doubts are of little consequence; since if among so many 

 animals resembling each other in their general characters, and 

 often indeed apparently belonging to the same genus, the lumi- 

 nous property was certainly proved to exist in some, it probably 

 existed equally in all ; as there seems no reason to exclude any, 

 or to suppose it especially possessed by one. On this subject, 

 however, other naturalists must be allowed to judge for them- 

 selves ; and those who are inclined to pursue the same train of 

 investigation will probably complete the evidence respecting 

 some where it is here left doubtful. 



I may now therefore conclude this subject by remarking, that, 

 from the investigation of last summer, I have added upwards 



