448 PHlLOSOPHldAL TRANSACTIONS. [aNNO I67O. 



that the retina was about half a line in thickness, and not half a line precisely; 

 which showed I had not measured it exactly; but if it were but a quarter of a 

 line or less, it suffices that it had enough for the effect I did attribute to it. 



You see then. Sir, that hitherto your objections have been able but very lightly 

 to shake my opinion, and that the transparency of the retina is well enough 

 established. Let us come now to the proof I make of the want of vision on the 

 basis of the optic nerve. It must first of all be agreed, that in this experiment 

 almost all men lose sight of an entire circle of white paper, whose diameter is 

 about the Qth or lOth part of its distance from the eye. Now the visual 

 triangle, whose basis is the diameter of the circle, and whose top is the centre 

 of vision, is proportioned to the triangle, whose basis is the diameter of the 

 picture of the circle on the bottom of the eye, and the top, the same centre of 

 sight, which centre being distant six or seven lines from the basis of the optic 

 nerve, whose breadth is about ^ of a line, this basis also will be about the 9th 

 or 10th part of its distance from the centre of sight, and by the principles of 

 optics, the image of the circle of the white paper, falling on the basis of the 

 optic nerve, will cover it precisely ; and because the paper then wholly disappears, 

 it follows, that all the basis of the optic nerve is insensible of light ; whence I 

 conclude, that the choroides is the principal organ of sight; and that the retina 

 is not, seeing it is placed in that part, and is there apparently disposed in like 

 manner as to the rest of the bottom of the eye. 



To elude the force of this argument, you allege other causes of this fault of 

 sight ; the first two are almost like one another. But it seems to me that you 

 suppose them without any ground : for, as I said before, there are no such 

 filaments of nerves to be seen coming out of the basis of the optic nerve. The 

 other cause which you bring, is the trunk of the vessels which proceed from the 

 basis of the nerve. Yet you cannot deny but they are very small, and that it is 

 very hard to discern the little holes through which they pass, when the nerve is 

 cut off above its insertion into the eye; and because they often come out of the 

 basis by two several little holes, the diameter of each of which does not take up 

 the space of above the eighth part of the diameter of the basis ; it follows, that 

 if the rest of the nerve were sensible of light, we should not lose sight of a pa- 

 per of two inches diameter at most, at ten feet distance : and sometimes in fix- 

 ing one eye on a little piece of paper, two other very little ones, separated one 

 from the other, would disappear; which is contrary to experience ; for the de- 

 fault of vision is continued. 



Other observations I add, as follow : 



The first, which is very common, is, that the pupil dilates itself in the 

 shade, and contracts itself in a great light, and it is very hard to find the cause 



