Luminous Animals. 257 



ticable interval of time after' viewing them through the lens. 

 A few seconds are sufficient to cause the observer to forget the 

 exact figure of the parts which he is to delineate. 



The most convenient receptacle in which they can be placed 

 for examination is a rummer or conoidal glass, of such dimen- 

 sions as to contain half a pint. It is, in the first place, quite 

 necessary that they should be at liberty ; as it is only when in 

 motion that many of them can at all be discovered, and as the 

 peculiar nature of their motions, which, in all, are very different 

 and highly characteristic, is of great use in discriminating 

 individuals otherwise much resembling each other. It is true, 

 that this is productive of great inconvenience, from their pass- 

 ing so quickly out of the field of view; and thus it often 

 requires a long time and patiently repeated examinations, to 

 ascertain the exact figure of one individual. But it is impos- 

 sible to confine them in a drop of water, unless when absolutely 

 microscopic, without losing sight of their forms. In this case, 

 they come to a state of rest ; and their fins, legs, antennae, or 

 other fine parts, become invisible, generally collapsing close to 

 the body. Moreover the affection of light produced by the con- 

 tact of the animal with the surface or edge of the drop, or of 

 that of the drop with the glass on which it stands, totally 

 destroys distinct vision, and renders their forms quite unintelligi- 

 ble. A glass of smaller dimensions, such as a wine glass, is far 

 less convenient than that abovementioned ; as the smallness of 

 the bonvexity produces a much less useful spot of light. 



In many cases, where, from excessive activity, it is difficult 

 to catch these objects in the field of view for a sufficient time 

 to study their parts, I have found it useful to diminish their 

 powers of motion. This may be done by slightly warming the 

 water, by suffering it to stand for a few hours in the glass, or 

 by the addition of a small quantity of spirits, and probably of 

 other substances. But slight injuries are sufficient to kill them ; 

 and, as they then become invisible, the observer must be on 

 his guard not to exceed in the application of these means. 



From the necessity of using a large glass, and the freedom of 

 motion thence allowed, it is evident that a high magnifying 



