6 Dr. Brewster on the Vision of Impressions on the Retina. 



into the position B, the external object O remaining fixed. 

 The image of O upon the retina will now be raised from P 

 to Q in the elevated eye at B. Hence the object O will now 

 be seen in the direction QON, having descended, by the ele- 

 vation of the eye, from M to N. 



Let the eye be now brought back to its original position 

 A, and let the object O be the lamp with ground glass used 

 by Mr. Bell. The spectral impression will therefore be made 

 upon the retina at P, and will remain on that spot till it is 

 effaced. If the eye A is now raised to B, the impression will 

 still be at P in the elevated eye, and it will be seen in the di- 

 rection PB, parallel to PM, having- risen only /gth of an 

 inch, or the height through which the eye has been raised by 

 pressure. This small space is not very visible to an ordinary 

 observer, when his head is at liberty to move ; but if the 

 head is carefully fixed, the motion of the spectrum becomes 

 quite apparent. Hence it is obvious that Mr. Bell has been 

 first misled by not observing the motion of the spectrum, and 

 secondly, by supposing that the vision of an impression fol- 

 lowed the same law as the vision of an external object. 

 The difference between these two cases of vision which Mr. 

 Bell has overlooked, consists in this, that in ordinary vision 

 the object forms a new image upon a new part of the retina, 

 after the eye is pushed up ; whereas in spectral vision, the 

 original object has nothing to do after the eye is displaced, 

 the spectrum itself which retains its place on the retina be- 

 ing now the only object of perception. 



% The second fact announced by Mr. Bell is, that during 

 sleep, or upon the closing of the eyelids, the eyeball is in- 

 voluntarily turned up beneath the upper eyelid, and so far 

 even as to withdraw the pupil from the faint light which that 

 eyelid transmits. 



This singular result stands in direct contradiction to the 

 opinion of Soemmering and other anatomists, who consider 

 the eyeball as perfectly stationary when the eyelids are shut ; 

 but as Mr. Bell has deduced his opinion from direct experi- 

 ment, it requires to be strictly examined. I have frequently 

 and carefully repeated the experiment which he describes, 

 and I find that no such motion of the eyeball takes" place 

 upon shutting the eyelids ; but that, on the contrary, 

 they remain perfectly stationary. I am informed also by 



