SINGLE VISION. 55 



external situation of the spot, which was sug- 

 gested by the affection of that eye, would like- 

 wise be altered, and the spot by consequence 

 be seen double. As the event, however, was 

 contrary to my expectation, I began to suspect 

 some cause of fallacy had been overlooked, 

 which at length I thought might be this, that 

 the spot had been seen by that eye only whose 

 position was not disturbed, the violence, suf- 

 fered by the other, interrupting the due exer- 

 cise of its functions. To determine, therefore, 

 whether my conjecture was well founded or 

 not, I made another experiment, which is men- 

 tioned in the following article : 



3. Having looked steadily for some time at 

 the flame of a candle, with one eye only, I 

 directed afterward, with both eyes open, my 

 attention to the middle of a sheet of paper, a 

 few feet distant; the consequence of which 

 was, that a spot appeared upon it in the same 

 manner, as if I had viewed the flame with both 

 eyes, though somewhat fainter. My attention 

 remaining fixed upon the sheet, I now pushed 

 the eye, by which the spot was seen, succes- 

 sively upward and downward, to the right and 

 to the left, and in every oblique direction ; the 

 spot however never altered its position, but 

 kept constantly upon the middle of the appear- 

 ance of the paper, perceived by the undistorted 



