378 
Mr Charles Bell on the Motions of the Eye. 
is parallel to the direction in which it was before it was dls« 
placed, — a thing impossible. 
If you go to a distance from the candle and repeatedly press 
or strike the ball of the eye with the point of the finger, the 
light will seem to move and dance, through a considerable space, 
say a foot. Move close to the candle, and do the same, and the 
light will not seem to move half an inch. The reason is this. 
Let A (fig, 2.) be the centre, on which the eyeball revolves, 
its revolution will be attended with an appearance of motion in 
the body B, through a certain portion of the smaller sphere of 
vision C, D. But if the body be distant as E, it will appear 
to move, as the figure indicates, through a corresponding portion 
of a larger sphere of vision E G. Exactly the reverse would 
take place if the axis of the eye moved from one parallel to an- 
other : That is to say, the motion of the eyeball, which gives 
the apparent motion to the object, would be great, if the object 
were nearer the eye, and would be as nothing if removed to a 
distance. Thus we are inibrmed, that when the eye-ball is 
moved whether covered by the eyelids or not, it revolves upon 
its centre ; and, therefore, the diagram of Dr Brewster, however 
ingeniously reared, is founded on antecedent propositions which 
are false. Having cleared a\vay this obstruction, I resume my 
original position. 
If aa^ impression is made upon the retina, and the eye be 
shut or in darkness, and a voluntary muscular effort be made 
with tjie eye, the image or phantom will seem to change its 
place in a direction corresponding with the effort. But if the 
eye-ball suffer any change of place or revolution from any other 
cause than voluntary effort, the image or phantom will retain 
its place, the mind taking no. cognizance of that motion. 
The next observation of mine, which Dr Brewster opposes, 
regards the motion of the eye-lids. When we wink or close the 
eye to lubricate the cornea, or render it fit for vision, I have as- 
serted, that, at the same instant, the eye-ball is turned up, and 
that, without this, it would not be thoroughly lubricated and 
washed, and that the fluid would be left upon the cornea suf- 
fusing the vision. 
Dr Brewster observes, Unfortunately for these views, the 
clearing away of the lubricating fluid, which is left in the groove 
