Dr. Brewster on the Vision of Impressions on the Retina. 3 



cause, by the involuntary muscles or by pressure from without, we shall 

 have no corresponding change of sensation. 



If we make the impression on the retina in the manner described, 

 and shut the eyes, the image will not be elevated, although the pupils be 

 actually raised, as it is their condition to be when the eyes are shut, be- 

 cause there is here no sense of voluntary exertion. If we sit at some 

 distance from a lamp, which has a cover of ground glass, and fix the eye 

 on the centre of it, and then shut the eye and contemplate the phantom 

 in the eye ; and if, while the image continues to be present of a fine 

 blue colour, we press the eye aside with the finger, we shall not move 

 that phantom or i?uage, although the circle of fight produced by the pres- 

 sure of the finger against the eyeball moves with the motion of the fin- 

 ger. 



May not this be accounted for in this manner : The motion produced 

 in the eyeball not being performed by the appropriate organs, the volun- 

 tary muscles, it conveys no sensation of change to the sensorium, and is 

 not associated with the impression on the retina, so as to affect the idea 

 excited in the mind ? It is owing to the same cause, that, when look- 

 ing on the lamp, by pressing one eye, we can make two images, and we 

 can make the one move over the other. But if we have received the im- 

 pression on the retina so as to leave the phantom visible when the eye- 

 lids are shut, we cannot, by pressing one eye, produce any such effect. 

 We cannot, by any degree of pressure, make that image appear to move ; 

 but the instant that the eye moves by its voluntary muscles, the image 

 changes its place ; that is, we produce the two sensations necessary to 

 raise this idea in the mind ; we have the sensation on the retina com- 

 bined with the consciousness or sensation of muscular activity." — Phil. 

 Trans. 1823, p. 177—180. 



The passage now quoted contains three important results : 



1. That when an impression is made upon the retina by 

 6trong light, this impression, in the form of a coloured spec- 

 trum, remains absolutely fixed and immoveable, if the eye- 

 ball is moved by the pressure of the finger, or by any other 

 external cause than that of the voluntary muscles of the eye- 

 ball. 



% That during sleep, or upon the closing of the eyelids, 

 the voluntary muscles resign their office, and the involuntary 

 muscles draw the pupil under the upper eyelid. 



3. That during this involuntary motion or displacement of 

 the globe of the eye, the spectral impression continues abso- 

 lutely fixed and immoveable. 



From these three results, Mr. Bell draws the highly impor- 

 tant conclusion, that '< it is by the consciousness of the de- 

 gree of effort put upon the voluntary muscles, that we know 

 the relative position of an object to ourselves,'" or that " the 



