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ideas so rich and varied, is liable to betray us into many errors. Jt may 

 be doubted, whether it gives the notion of distance, since the boy couch- 

 ed by Cheselden, conceived every thing he saw to touch his eye. It ex- 

 poses us to false judgments on the form and size of objects 5 since, agree- 

 ably to the laws of optics, a square tower seen at a distance, appears 

 to us round ; and very lofty trees seen also very far off, seem no taller 

 than the shrubs near us. A body, moving with great rapidity, appears 

 to us motionless, Sec. It is from the touch, that we gain the correction 

 of these errors, which Condillac, in his Treatise on Sensation, has per- 

 haps exaggerated. 



CXX. The organ of sight, in different animals, varies according to the 

 medium in which they live; thus, in birds which fly in the higher regions 

 of the air, there is an additional and very remarkable eye-lid: this is par- 

 ticularly the case with the eagle, which is thus enabled to look at the sun, 

 and with night birds, whose very delicate eye it seems to protect fromHhe 

 effects of too strong a light. In birds, likewise, there is a copious secre- 

 tion of tears, the medium in which they live causing a considerable eva- 

 poration. The greater part of fishes, on the contrary, have no jmq^eable 

 eye-lid, and their eyes are not moistened by tears, as the water in which 

 they are immersed, answers the same purpose, In some fishes, however, 

 the eyes are smeared with an unctuous substance calculated to prevent 

 the action of the water on the organ. 



The globe of the eye in birds, is remarkable by the convexity of the 

 cornea, which is, sometimes, a complete hemisphere, hence it possesses 

 a considerable power of refraction. This power of refraction, appears to 

 be very weak in fishes, the fore part of their eyes being very much flat- 

 tened; but the water in which they live made it unnecessary, that they 

 should have an aqueous humour, for, the density of this fluid being near- 

 ly the same as that of water, it would not have produced any refraction : 

 besides, being, in sea-fish, of inferior density to that of salt-water, it would 

 have broken the rays of light, by making them diverge from the perpen- 

 dicular. In fact, the refractive power of a medium is never but a relative 

 quantity ; the degree of refraction is not determined by the density of the 

 medium, but by the difference of density between it and the medium that 

 is next to it. To make up for the flatness of the cornea occasioned by the 

 small quantity, or even by the absence, of aqueous humour, fishes have a 

 very dense and spherical crystalline humour, the spherical part of which 

 forms a part of a small sphere. 



The eyes of birds, whose cornea is thrust out by a very copious aque- 

 ous humour, possess, in consequence of the presence of this fluid, a very 

 considerable power of refraction ; the air, in the higher regions of the air, 

 owing to its extreme rarefaction, being but little calculated to approxi- 

 mate the rays of light. 



The pupil admits of greater dilatation in the cat, in the owl, in night 

 birds, and in general in all animals that see in the dark. The sensibility 

 of the retina appears, likewise, greater in those animals ; several of them 

 appear incommoded by the light of day, aad never pursue their prey, but 

 in the most obscure darkness. 



The crystalline humour of several aquatic fowls, as the cormorant's, is 

 spherical like that of fishes, and this is not, as will be mentioned here- 

 after, the only peculiarity of structure in these kind of amphibious ani- 

 mals. Lastly, the choroid of some animals, ifrore easily separated into 

 two distinct lamina, than that of man, presents, at the bottom of the eye, 



