266 On the Optical and Physiological Discoveries 



spot, and, consequently, upon those of external objects, which, as far as I 

 know, have not been mentioned by any other person. 



" 1. The spot is always seen single, whether the surface, upon which it 

 is projected, be touching the face, or at the greatest distance from us ; and 

 the reason is plain. For the parts of the retinas, by whose affections from 

 the luminous body it is occasioned, are those likewise which receive the 

 pictures of objects placed at the intersection of the optic axes ; and as such 

 objects always appear single, so must also the spot. The fact indeed is so 

 open to observation, and its cause so easily shown, that I should scarcely 

 have thought of mentioning it had not Dr Darwin lately told us that the 

 spot is seen double as often as the eyes are directed to an object more or 

 less distant than the luminous body which gave rise to it. With respect 

 to our different assertions upon this point, I shall only say, that I have 

 made the experiment, I believe, upwards of an hundred times, uniform- 

 ly with the same result ; and that, if the spot ever appears double, this 

 must be from some cause very wide of a change in the mutual inclination 

 of the optic axes, to which he attributes it. 



" 2. The spot not only appears single in every ordinary position of the 

 optic axes, but cannot even be made to appear double by any means what- 

 ever. If it be projected, for example, upon a piece of white paper, who- 

 ever makes the trial, will find that, although on pressing one eye upward 

 or downward, or to either side, the paper will be seen double, yet the spot 

 will always appear single, and to possess its former place on the paper, as 

 seen by the eye which is not disturbed. Before I knew the result of this 

 experiment, I had imagined, that the position of one eye being forcibly al- 

 tered, the external situation of the spot, which was suggested by the af- 

 fection of that eye, would likewise 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 had not been disturbed, the violence suffered by the 

 other interrupting the due exercise of its functions. To determine, there- 

 fore, whether my conjecture was well founded or not, I made another ex- 

 periment, which is mentioned in the following article. 



" 3. Having looked steadily for some time at the flame of a candle, with 

 one eye only, I directed afterwards, 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 

 received the flame with both eyes, though somewhat fainter. My atten- 

 tion remaining fixed upon the sheet, I now pushed the eye, by which the 

 spot was seen, successively upward and downward, to the right and to the 

 left, and in every oblique direction ; the spot however, never altered its po- 

 sition, but kept constantly upon the middle of the appearance of the paper 

 perceived by the undistorted eye, though the appearance of the paper to the 

 distorted eye was always separate from the former, and the sheet conse- 

 quently seen double. My conjecture, therefore, was proved to be ill ground- 

 ed, and all suspicions of fallacy in the former experiment ceased. 



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