VOL. v.] PHILOSOPHICAL TRANSACTIONS. 44Q 



of this involuntary motion, but by the hypothesis, that the choroides is sensible 

 of light; for then it is easy to conceive, that being hurt by too strong a vision, 

 it may dilate or contract its fibres, which have one continuity with those of the 

 forepart of the uvea, so that it can contract its aperture, and when not hurt, 

 relax it again: whereas if the retina be supposed to be the organ of sight, it will 

 be very difficult to explain how this contraction is made. 



The second is that of the glass bottle full of water, when a lighted candle is 

 placed near its focus; for it is easy to prove that if one hold his hand between 

 the candle and the bottle, he will feel more heat than if he held it in the reci- 

 procal focus, that is to say, the place where the rays, which have passed through 

 the bottle, make a great image of the flame of the candle appear inverted upon 

 a white surface opposed to it; for I draw this consequence, that the image of a 

 candle which is painted on the choroides of a dog, as I have proved to you, 

 makes a much greater impression on the retina of the dog than on that of him 

 that looks on it, and sees it very bright; whence I conclude, that if the retina 

 were the organ of sight, this dog would not see the objects indifferently en- 

 lightened which are about the candle, although at three or four feet distance, 

 because they would receive a great deal more impression from this reflection, 

 and that a greater sensation obliterates a less; which is contrary to experience, 

 and it is not at all likely there should be such a defect in the sight of animals. 



The third is, that the eyes of birds are so framed, that the optic nerve, after 

 its insertion into the eye, is inflected, and extends itself on the concavity of the 

 sclerotis, about the breadth of two or three lines, more or less, according to the 

 bigness of the eyes ; and the length of this reflection is covered by the choroides, 

 leaving but one little white streak in the middle, from whence the retina takes 

 its original, which extends itself on the choroides through all the bottom of the 

 eye; but it is covered on the side of this white streak with a little black mem- 

 brane, as long as the inflexion of the nerve, and almost as broad, which proceeds 

 from the pia mater, and is as it were an appendix of the choroides ; and if you 

 consider the situation of this membrane, you will find it is near the axis of sight, 

 and that the rays of the objects which the birds look on with both eyes fall pre- 

 cisely upon it after their refraction. Since then, in the place where vision ought 

 to be strongest, the retina is covered, and that no man doubts but birds are 

 more clear-sighted than other animals, you ought to acknowledge, that the re- 

 tina is not the principal organ of vision, but that that pre-eminence belongs to 

 the choroides.* 



* The arguments here -employed are ingenious but by no means convincing. Some of the ana- 

 tomical assertions are erroneous. 



VOL. I. 3 L 



