1 78 Mr. Bell on the motions of the eye, &c. 
On the other hand, there is an inseparable connection be- 
tween the exercise of the sense of vision and the exercise of 
the voluntary muscles of the eye. When an object is seen, 
we enjoy two 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 conscious- 
ness of the degree of effort put upon the voluntary muscles, 
that we know the relative position of an object to ourselves. 
The relation existing 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 voluntary muscles of the eye-ball, 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 eye-lids 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 position of an object having 
relation to the activity of the muscles. 
We may also show, by varying this experiment, that an 
agitated state of the muscles, or a state of action where the 
muscles are at variance or confused, affects the idea of the 
