VOL. II.] PHILOSOPHICAL TRANSACTIONS. 245 



on the contrary, if it be in the choroides that vision is made, it seems evident 

 that the reason why there is none on the optic nerve, is because that membrane 

 (the choroides) parts from the edges of the said nerve, and does not cover the 

 middle as it does the rest of the bottom of the eye. 



Tlie following is M. Pecquet' s Answer to the above. 



Every one wonders that no person before you has been aware of this priva- 

 tion of sight, which every one finds, now you have given notice of it. But as 

 to the result you draw from this discovery, I see it not sufficiently cogent to 

 abandon the opinion of the retina being the principal organ of vision. For 

 it will be sufficient now to take notice, that at the place of the optic nerve there 

 is something that may very well cause this loss of the object. There are the 

 vessels of the retina, the trunks of which are large enough to intercept the 

 vision. These vessels, which are only the ramifications of the veins and arteries, 

 are derived from the heart, and having no communication with the brain, they 

 cannot carry thither the species of the objects. If therefore the visual rays, 

 issuing from an object fall on these vessels at the place of their trunk or main 

 body, it is certain that the impression made thereby will produce no vision, 

 and that the picture of that object will be deficient ; as when on a white paper 

 in an obscure chamber there is some black spot, or in it some hole of a con- 

 siderable size : for the more sensible this blackness or hole is, the more of the 

 image of the object it intercepts from our eyes. It is not so in respect of the 

 small ramifications that issue from those trunks, and shoot into the retina. 

 For if they be met with at the place of the bottom of the eye where vision is 

 made distinct, they will not render the image of the object deficient, because 

 they are so small as not to be sensible. Thus it is, that in looking-glasses, 

 when they want lead or tin in any place large enough to be perceived, the 

 image we there see appears to have a hole ; which happens not when there is 

 but so small a one as might be made by the point of a needle. 



Thus much being observed as to the deduction made from this experiment, 

 I shall further note, that that paper, the sight whereof is lost, must be further 

 off or nearer, according to the diversity of the structure of eyes. For some lose 

 this paper at the distance of two feet, some at a less, others at a greater dis- 

 tance ; some lose it a little higher, others a little lower, according as the trunks 

 of the vessels are situated in respect of the optic nerve ; and some lose more 

 of it than others, according as those vessels are larger or smaller. And because 

 it is hard to determine precisely the place where the object is lost in all sorts of 

 eyes, we have ground to believe that this loss is not always made on the extent 

 of the nerve where the retina is, but sometimes on the side of it where the cho- 



