VOL. v.] PHILOSOPHICAL TRANSACTIONS. 443 



Place by night a lighted candle very near your eyes; and cause a dog 8 or 10 

 paces distant from tlie candle to look upon you, then you will see in his eyes a 

 light sufficiently bright, which I hold to proceed from the reflection of the light 

 of the candle, the image of which is painted on the choroides of the dog, which 

 having much whiteness and lustre, causes this very strong reflection ; for if it 

 proceeded from the crystalline or retina, the same appearance would be seen in 

 the eyes of men, birds, and other animals, who have the choroides black, which 

 is not found so by us. It is therefore manifest by this experiment, that the 

 luminous rays pass with considerable force as far as the choroides, and that the 

 retina receives very little impression. Now this appearance is made thus: the 

 little picture of the candle which is upon the choroides of the dog, (where is the 

 focus of the crystalline and other humours together) sending rays back through 

 these humours, makes its reciprocal focus towards the candle, and by conse- 

 quence the eyes which are near to the point, where these rays reunite, ought 

 to see the crystalline of the dog very much illuminated. 



I believe that darkness is not absolutely necessary for vision, but only for a 

 strong vision : nor that the picture of objects ought to be expressed on the 

 organ of vision ; for, it is sufficient that the rays of each point of the objects be 

 reunited in a distinct and separated point, according as they answer one another; 

 and you will easily agree, that as a convex glass makes the image of the sun ap- 

 pear on white paper with a great deal of brightness and light, and on black paper 

 very obscurely ; although black paper, which soon takes fire, receives a great 

 deal more impression than white; so the rays of illuminated objects do reunite 

 on a whitish choroides, and express there a visible picture, and on a dark cho- 

 roides a very obscure one, and which cannot be seen ; but then the impression 

 is also much stronger in the black than in the white ; and this is the cause why 

 men and birds see better and more distinctly than the greatest part of other 

 animals ; for their choroides being black, and by consequence very sensible of 

 light, they contract much their pupil or sight-hole of the eye, which makes the 

 rays that pass there, from each point of the object, to be all very near the axis 

 of the crystalline, and to reunite more exactly in a point than in the eyes of 

 most other animals, which have their choroides white towards the axis of sight, 

 and by consequence less sensible of light, who in recompense can very much 

 dilate the pupil of their eyes, when they stand in need of a great light; but also 

 their sight is not so distinct, because the rays, which fall on the extremity of the 

 crystalline, intersect the axis too near in their refraction. 



It is true, that to supply in some sort this defect, they have a little crystalline 

 in the middle of the great one, and this little crystalline being of a more spiss 

 consistence than the great one, its refraction is also more strong, and makes the 



