17 9 
Mr. Bell on the motions of the eye , &c. 
image. If we look on the luminous body so as to make this 
impression on the retina, and then cover the face so as to 
exclude the light, keeping the eye-lids open, and if we now 
squint, or distort the eyes, the image which was vividly im- 
pressed upon the retina instantly disappears as if it were 
wiped out. Does not this circumstance take place, because 
the condition of the muscles thus unnaturally produced, being 
incongruous with the exercise of the retina, disturbs its ope- 
ration ? 
If we move the eye by the voluntary muscles, while this 
impression continues on the retina, we shall have the notion 
of place or relation raised in the mind ; but if the motion of 
the eye-ball be produced by any other cause, by the involun- 
tary 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, because 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 image, although 
the circle of light produced by the pressure of the finger 
against the eye-ball moves with the motion of the finger. 
May not this be accounted for in this manner : the motion 
produced in the eye-ball not being performed by the appropri- 
ate organs, the voluntary muscles, it conveys no sensation of 
