300 PHILOSOPHICAL TRANSACTIONS. [aNNO 1778. 



phers, whether the choroid coat of the eye or the retina was the immediate 

 organ of vision, which had lately been revived in some measure in Dr. Priestley's 

 valuable history of Light and Colours ; and it was then thought by one party in 

 this dispute, that the defect of the choroid coat, where the optic nerve enters the 

 eye, was the cause of this want of vision in that part. But the following obser- 

 vation shows beyond a doubt the fallacy of this supposition : the diameter of the 

 optic nerve, at its entrance into the eye, is about -^ of an inch, and the perfora- 

 tion of the choroid coat, through which it passes, must of necessity be of the 

 same diameter : now the dark, spot, which is seen in objects opposed to the centre 

 of the optic nerve, if it was occasioned by the deficiency of the choroid coat, 

 should, at 9 inches distance from the eye, be 54 times the diameter of this 

 aperture, or Q inches in diameter ; whereas Dr. D. found by experiment, that a 

 paper of 1 inch in diameter could not be totally concealed at O inches distance 

 from his eye : and M. Le Cat by accurate observations found, that the insensible 

 part of his eye was but between the 30th and 40th part of an inch in diameter. 

 This experiment is so easily made, that it can be attended with no fallacy ; and 

 at the same time that it shows that the insensible spot, where the optic nerve 

 enters the eye, is not owing to the deficiency of the choroid coat, entirely sub- 

 verts the opinion of the choroid coat being the organ of vision ; for vision exists 

 where the choroid coat is not. 



Nor is the insensibility of the centre of the optic nerve owing to the ingress 

 of the arteries along with it into the eye ; for a large branch of this artery runs 

 along the bottom of the eye, where vision is most distinct, and because all this 

 artery is covered with the expanse of the retina on the external side of it. Mr. 

 Savage made an experiment for another purpose, which however shows, that the 

 optic artery, where it is branched under or through the retina, does not much 

 disturb the power of vision. It is this : if you look on a white wall on a lumi- 

 nous day, with the sun shining on the wall only by its reflected ligiit, you will 

 discern the parts of the wall become darker and lighter at every pulsation of the 

 optic artery. This darker and lighter appearance is like net-work, and not 

 uniform like the wall itself; but the whole, though rather darker while the 

 diastole of the artery compresses the retina, is yet distinctly visible. 



The following circumstance seems to give rise to the insensibility of the 

 central part of the optic nerve at its ingress into the eye, which he had observed 

 m several calves' eyes. The point of a pair of scissars was introduced behind 

 the ciliary circle, and the whole of the cornea, aqueous humour, iris, and 

 crystalline, being removed, the retina was beautifully seen through the vitreous 

 humour somewhat magnified. On exposing this to the sun-shine, and inspect- 

 ing it with nicety, a white filament, about the lOth of an inch in length, arising 

 from the centre of the optic nerve, was seen ascending straight upwards into 



