VOL. v.] PHILOSOPHICAL TRANSACTIONS. 447 



in the retina; and infallibly, if there were any nerves, you might find them in 

 the same manner, and they would stop the pin, since they are as hard and firm 

 as the little arteries; and when you say, that one may distinguish these filaments 

 in the water, because the rest of the retina disappears, that is repugnant to expe- 

 rience and to what you have said before, viz. that the retina may be seen all 

 white in the water, and without transparency ; and you ought to show these 

 filaments, or else we shall take them for a thing invented at pleasure. 



You bring after that two experiments, the first of which is, that if an aper- 

 ture be made on the upper part of the eye, we may discover the picture of the 

 objects on the anterior surfkce of the retina ; but if this aperture be made in the 

 white of the eye, the aqueous humour will run to the tunica cornea, and make 

 wrinkles, which will hinder the picture from being distinct; besides, he that 

 looks in at this aperture, will hinder the rays of the object from passing into 

 the eye, and he shall see nothing there but his own image. But if you mean, 

 that the tunica cornea should be wholly taken off, there will not then be dis- 

 tance enough between the crystalline and the retina to make the picture dis- 

 tinct. In conclusion, I do not believe this experiment can be made, much less 

 that it is to be discerned, whether this picture be formed on the anterior or 

 posterior surface of the retina, since the thickness of it is less than half a line, 

 or the 24th part of an inch; and there is reason to believe, that you have trusted 

 to the report of some other concerning this experiment ; or that you have be- 

 lieved the images which appear in the eyes to be painted on the retina, whereas 

 they proceed from the reflection made on the exterior of the cornea. 



Your second experiment is true and easy to be made, but according to you it 

 were impossible ; for, since you hold that it is on the anterior part of the retina 

 that the picture is seen, and that you have elsewhere said, that one cannot well 

 see through this membrane, it follows that you cannot see this picture through 

 the thickness of the retina; but because I believe there remains transparency 

 enough in that part of the retina which is not exposed to the air, I doubt not 

 but the picture may be seen on the posterior part of it, after it is become suffi- 

 ciently opaque, though in a living animal this picture passes as far as the cho- 

 roides, as it has been already proved; and if the retina itself were taken away, 

 and there remained only the vitreous humour, you might notwithstanding see 

 the inverted picture of the windows toward the bottom of the eye, if you held 

 it at the farther side of the room, in the same manner as the picture is to be 

 seen in the focus of the spherical glass-bottle filled with water, though it seem 

 to be on the exterior surface of the glass, and by consequence this experiment 

 proves not the opacity of the retina. 



In your third objection you quote my observation erroneously, for I wrote 



