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



ty, I trust it will not be supposed that I am engaging their at- 

 tention to a subject of a controversial nature. I have no incli- 

 nation to offer any criticisms, or make any comments upon 

 those parts of Mr. Bell's paper, which are open to controver- 

 sy. My only object is to establish certain scientific facts and 

 laws of vision which have been misunderstood or perverted ; 

 and I shall but ill perform the task I have undertaken, if I 

 leave the subject in any doubt, or fail to impress upon those 

 who hear me, the same conviction of their certainty which I 

 entertain myself. 



In order that the facts and doctrines maintained by Mr. 

 Bell may not be misinterpreted, I shall state them in his own 

 words. 



" When the eye Is at rest, as in sleep, or even when the eyelids are 

 shut, the sensation on the retina being then neglected, the voluntary 

 muscles resign their office, and the involuntary muscles draw the pupil un- 

 der the upper eyelid. This is the condition of the organ during perfect 

 repose. 



On the other hand, there is an inseparable connexion between the ex- 

 ercise of the sense of vision, and the exercise of the voluntary muscles 

 of the eye. When an object is seen we enjoy too senses ; there is an 

 impression upon the retina ; but we receive also the idea of position or 

 relation, which it is not the office of the retina to give. It is by the 

 consciousness of the degree of effort put ujwn the voluntary muscles that 

 we know the relative position of an object to ourselves. The relation ex- 

 isting between the office of the retina and of the voluntary muscles, may 

 be illustrated in this manner. 



Let the eyes be fixed upon an illuminated object, until the retina be 

 fatigued, and in some measure exhausted by the image, then closing the 

 eyes the figure of the object will continue present to them : and it is 

 quite clear that nothing can change the place of this impression on the 

 retina. But notwithstanding that the impression on the retina cannot 

 be changed, the idea thence arising may. For, by an exertion of the vo- 

 luntary muscles of the eyeball the body seen will appear to change its 

 place, and it will, to our feeling, assume different positions according to the 

 muscle which is exercised. If we raise the pupil we shall see the body 

 elevated, or if we depress the pupil, we shall see the body placed below 

 us; and all this takes place while the eyelids are shut, and when no new 

 impression is conveyed to the retina. The state of the retina is here 

 associated with a consciousness of muscular exertion ; and it shows that 

 vision, in its extended sense, is a compound operation, the idea of posi- 

 tion of an object having relation to the activity of the muscles. * * • 

 If we move the eye by the voluntary muscles, while this impression con- 

 tinues upon the retina, we shall have the notion of place and relation rais- 

 ed in the mind; but if the motion of the eyeball be produced by any other 



