from the Lenses of' Fishes. 99 



occur in such abundance and variety in the eyes of the diffe- 

 rent classes of the animal kingdom. 



As a high magnifying power is, under such circumstances, 

 indispensable, we are of course limited to the use of the 

 smaller lenses of animals, and perhaps also to those which 

 have nearly a spherical form. The lenses of fishes are, there- 

 fore, most likely to answer the object which we have in view, 

 both from their being generally of a spherical form, and from 

 their superior density, which renders them less liable to in- 

 jury than those of birds and quadrupeds, when they are in a 

 state of preparation for use. 



As the lenses of fishes, however small, are not truly sphe- 

 rical, but are generally of a spheroidal form, it becomes abso- 

 lutely necessary, previous to their use, that we determine the 

 optical axis of the lens, or the axis of vision of the eye from 

 which it is taken, and place the lens in such a manner that its 

 axis is parallel to the axis of our own eye. In no other di- 

 rection but this is the albumen or matter, which composes the 

 lens, symmetrically disposed round a given line; — and in no 

 other direction does the gradation of density, by which the 

 spherical aberration is corrected, preserve a symmetrical re- 

 lation to the axis of vision. 



When the lens, therefore, which we shall suppose that of a 

 small Par, freshly taken from the river, has been removed, along 

 with the vitreous humor from the eye, by cutting with a pair 

 of sharp scissars an opening in the sclerotic coat, it should 

 be placed upon a piece of fine silver paper, previously freed 

 from all the little adhering fibres. The absorbent nature of 

 the paper will assist in removing all the vitreous humor from 

 the lens ; and when this is carefully done, there will still re- 

 main round or near the equator of the lens a black ridge, 

 consisting of the processes by which it was suspended in the 

 eye. This black circle points out the true axis of the lens, 

 which is perpendicular to it. 



When the small crystalline has been freed from all the ad- 

 hering vitreous humor, the capsule in which it is kept will have 

 a surface as fine and smooth as if it were a pellicle of fluid. 

 It is then to be rolled upon a piece of silver paper, by push- 

 ing it about with another piece of silver paper, and afterwards 

 dropped from this paper into a cavity c d, (Plate II. Fig. 13.) 



