180 Mr. Bell on the motions of the eye , &c. 
change to the sensorium, and is not associated with the im- 
pression on the retina, so as to affect the idea excited in the 
mind ? It is owing to the same cause that, when looking 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 re- 
ceived the impression 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 sensa- 
tion on the retina combined with the consciousness or sensa- 
tion of muscular activity. 
These experiments and this explanation of the effect of the 
associated action of the voluntary muscles of the eye-ball, 
appear to me to remove an obscurity in which this subject 
has been left by the latest writers. In a most scientific ac- 
count of the eye and of optics, lately published, it is said on 
this question, “ we know nothing more than that the mind 
residing, as it were, in every point of the retina, refers the 
impression made upon it, at each point, to a direction coin- 
ciding with the last portion of the ray which conveys the 
impression.” The same author says “ Kepler justly ascribed 
erect vision from an inverted image to an operation of the 
mind by which it traces the rays back to the pupil, and thus 
refers the lower part of the image to the upper side of the 
eye.” What can be here meant by the mind following back 
the ray through the humors of the eye ? It might as well 
follow the ray out of the eye, and like the spider feel along 
