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



notion of place or relation is raised in the mind ;"" and hence 

 he explains the old paradox of an inverted picture upon the 

 retina producing the appearance of an erect object. 



In estimating the value of this singular conclusion, we 

 shall first admit its truth, as well as the correctness of the 

 facts from which it is deduced, in order to form some notion 

 of the consequences in which it will involve us. 



Since the notion of place or relation depends solely on the 

 consciousness of exerting the voluntary muscles of. the eye- 

 ball, let the observer, with a spectral impression on his retina, 

 close his eye, and turn round his head either in a vertical or 

 a horizontal plane, by the muscles of his neck alone. It will 

 now be found, that the spectrum follows the motion of the 

 head ; and hence we must conclude, that the notion of place 

 or relation depends on the exercise of the muscles of the 

 neck, as those of the eyeball have been entirely at rest. 



But as there may exist some undiscovered sympathy be- 

 tween the muscles of the neck and those of the eyeball, let 

 the observer, with his eyes closed be now placed upon a stool, 

 to which an assistant communicates a rotatory motion 

 through the intermedium of a leathern belt. In this case 

 also, it will be found that the spectrum revolves with the 

 stool in the same manner as if the eyeball had performed the 

 same angular motion by the action of its voluntary muscles. 

 Hence we must conclude, that the notion of place or relation 

 depends on the muscles of the assistant's arm, conveyed by 

 some sympathetic action to the observer's eye along the lea- 

 thern belt ; a result so inadmissible, that, to use the sentence 

 which Mr. Bell directs against the illustrious Kepler, '• The 

 mind might as well follow the ray out of the eye, and like 

 the spider, feel along the line." 



In order to view this subject under another aspect, let us 

 suppose that, by cutting the voluntary muscles, the eyeball is 

 left to float in its socket ; or, what is the same thing, that 

 these muscles have lost their power of giving motion to the 

 eyeball. In such a case, will the eye retain its notions of 

 place or relation? or will it lose them entirely ? It is quite 

 clear that the impression of external objects on the retina 

 will not be affected by this condition of the voluntary mus- 

 cles ; and therefore it follows, that if the notion of place is 



