OF OPTICS. 243 



the impulse of the rays trora the visible object upon LECT. 

 the retina of the eye, and forming the image of the ob- v J^ I !l 

 ject thereon, and that the retina is only the expansion 

 of the optic nerve all over the choroides ; it should 

 seem surprising, that the part of the image which falls 

 on the optic nerve should render the like part of the ob- 

 ject invisible ; especially as that nerve is allowed to be 

 the instrument by which the impulse and image are con- 

 veyed to the common sensory in the brain. But this 

 difficulty vanishes, when we consider that there is an 

 artery within the trunk of the optic nerve, which en- 

 tirely obscures the image in that part, and conveys no 

 sensation to the brain. 



That the part of the image which falls upon the middle 

 of the optic nerve is lost, and consequently the corres- 

 ponding part of the object is rendered invisible, is plain 

 by experiment. For, if a person fixes three patches, 

 A, B, C, upon a white wall, at the height of the eye, and 

 the distance of about a 

 foot from each other, 

 and places himself before 

 them, shutting the right 

 eye, and directing the 



left towards the patch C, he will see the patches 

 A and C, but the middle patch B will disappear. Or, if 

 he shuts his left eye, and directs the right towards B, he 

 will see both A and C, but B will disappear ; and if he 

 directs his eye towards B, he will see both B, and A, 

 but not C. For whatever patch is directly opposite to 

 the optic nerve N, vanishes. This requires a little prac- 

 tice after which he will find it easy to direct his eye, so 

 as to lose the sight of whichever patch he pleases. 



We are not commonly sensible of this disappearance, 

 because the motions of the eye are so quick and instan- 

 taneous, that we no sooner lose the sight of any part of an 

 object, than we recover it again ; much the same as in the 

 twinkling of our eyes, for at each twinkling we j\re 



2 R 



