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



diminished by reflexion from a convex surface, and let this 

 image be brought near the eye, so that the pencils of rays 

 which diverge from it may have their foci a great way be- 

 hind the retina. When the eye is open, the image of this 

 luminous point will be a circular disc of light, or a section of 

 the cone of rays formed by the refraction of the eye. If, when 

 looking at this circular disc, shown at A in Fig 2, we shut the 

 eyelids, and then open them gradually, examining at the same 

 time the appearance of the disc, we shall at first observe it 

 to have the compressed form shown at B, occasioned by the 

 ridge of fluid, and then gradually extending itself into its 

 regular circular form, an effect which may be produced at 

 once by the operation of winking ; the only one which nature 

 has combined with the ordinary motion of the eyeball for the 

 purpose of smoothing the outer surface of the cornea. 



In concluding these remarks, I cannot avoid expressing a 

 wish that Mr. Bell will re-examine his own observations, and 

 repeat with care those to which I have had occasion to re- 

 fer, before he proceeds to his ulterior object of establishing 

 upon such a basis an arrangement of the nerves of the eye, 

 and a distinction of them according to their uses. Such an 

 arrangement must be affected by the facts upon which it is 

 founded ; and the present advanced state both of human and 

 comparative anatomy, requires that all their classifications, 

 and particularly their most difficult ones, should not rest on 

 contested data, or be regulated by ambiguous principles. 



Before quitting this subject, I am desirous of stating to 

 the society some views connected with the preceding obser- 

 vations, and relating to a more recondite affection of the eye, 

 which it seems to receive through the agency of the mind. 



When the eye is not exposed to the impressions of exter- 

 nal objects, or when it is insensible to these impressions, in 

 consequence of the mind being engrossed with its own ope- 

 rations, any object of mental contemplation which has either 

 been called up by the memory,, or created by the imagina- 

 tion, will be seen as distinctly as if it had been formed from 

 the vision of a real object. In examining these mental im- 

 pressions, I have found that they follow the motions of the 

 eyeball exactly like the spectral impressions of luminous ob- 

 jects, and that they resemble them also in their apparent im* 



