APPLICATION OP THE ARGUMENT. 21 



stirved upon the figure of the crystalline compensating by 

 its roundness the density of the medium through which 

 their light passes : to which we have to add, that the eyes 

 of fishes, in their natural and indolent state, appear to be ad- 

 justed to near objects, in this respect differing from the hu- 

 man eye, as well as those of quadrupeds and birds. The 

 ordinary shape of the fish's eye being in a much higher de- 

 gree convex than that of land animals, a corresponding 

 diflference attends its muscular conformation, viz. that it 

 is throughout calculated ioY fiattening the eye. 



The iris also in the eyes of fish does not admit of con- 

 traction. This is a great difference, of which the proba- 

 ble reason is, that the diminished light in water is never 

 too strong for the retina. 



In the eel, [Plate III. fig. 5.] which has to work its head 

 through sand and gravel, the roughest and harshest sub- 

 stances, there is placed before the eye, and at some distance 

 from it, a transparent, horny, convex case or covering, 

 which, without obstructing the sight, defends the organ. 

 To such an animal, could any thing be more wanted, or 

 more useful? 



Thus, in comparing together the eyes of different kinds 

 of animals, we see, in their resemblances and distinction, 

 one general plan laid down, and that plan varied with the 

 varying exigencies to which it is to be applied. 



There is one property, however, comaion, I believe, 

 to all eyes, at least to all which have been examined,* 

 namely, that the optic nerve enters the bottom of the eye, 

 not in the ceutre or middle, but a little on one side; not 

 in the point wlsere the axis of the eye meets the retina, 

 but between that point and the nose. — The difference 

 which this makes is, that no part of an object is unperceiv- 

 ed by both evf^s at the same time. 



In considorinor vision as achieved by the means of an 

 image formed u the bottom of the eye, we can never re- 

 flect, without wonder, upon the smallness, yet correct'jess^ 

 of the picture, the subtlety of the touch, the fineness of the 

 lines. A 1 mdscape of five or six square leagues is hroujjht 

 into a space of half an inch diameter ; yet the multitude of 

 objects which it contains are all preserved ; are all discrim- 

 inated in their magnitudes, positions, figures, colours. The 



* The eye of the seal or sea-calf, I understand, is an exception,-— 

 Mem. Acad. Paris, 1701, p. 123. 



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