AN ESSAY ON 



eye, though the appearance of the paper to the 

 distorted eye, was always separate from the 

 former, and the sheet consequently seen double. 

 My conjecture, therefore, was proved to be ill 

 grounded, and all suspicion of fallacy in the 

 former experiment ceased. 



Now it is evident, from these two last experi- 

 ments, that the situation of the spot does not 

 depend upon the bare position of the eyes, or 

 else, in the former of them, it would have ap- 

 peared double, and in the latter, it would have 

 been moved from the middle of the paper, 

 when the only eye by which it was seen was 

 pushed from its place. Neither can it depend 

 upon the bare position of the muscles of the 

 eye, as these were also moved in the same ex- 

 periments ; nor upon any affection whatever of 

 the optic nerve. For since this last substance 

 is altogether passive, even in those motions of 

 the eyes which do occasion a change of the 

 spot's situation, every alteration, induced upon 

 the nerve by those motions, must be ultimately 

 ascribed to a change of its position ; and we 

 have seen, that similar changes of its position 

 have been produced by external violence, with- 

 out any alteration of the spot's situation. The 

 apparent situation of the spot being, therefore, 

 dependant upon none of these circumstances, 

 and being at the same time affected by the 



