377 
Mr Charles Bell on the Motions of the Eye. 
general information, I begin to think there was more novelty in 
proving this fact than I at first conceived. 
I shall, in the second place, examine the observations of Dr 
Brewster, which he introduces to the philosophical public as 
“ scientific facts. 
In the papers referred to, I had shewn, that, if w^e look upon 
an illuminated object until its image is permanent in the bottom 
of the eye, the phantom moves with the motion of the eye, if 
that motion be performed'by the voluntary muscles of the eye- 
ball. But if the eye-ball be moved by the finger, the phantom 
will remain stationary, while the ball itself is in passive motion. 
Dr Brewster proposes to prove, that the spectrum or phan- 
tom is by no means immoveable, and that it moves the precise 
degree it ought to move. The diagram by which he would per- 
suade us of tills is given in Plate XI. Fig. 1. Let A be the 
eye of the observer, and O an internal object, whose image at P 
is seen along the axis of vision POM. Let the eye be pushed 
upwards, suppose y^^th of an inch into the position B, the ex- 
ternal object O remaining fixed. The image of O, upon the re- 
tina, will now be raised from P to Q, in the elevated eye at B. 
Hence the object will now be seen in the direction QON, having 
descended by the elevation of the eye from M to N. 
Let the eye be now brought back to its original position A, 
and let the object O be the lamp, with ground glass, used by 
Mr Bell. The spectral impression will therefore be made up- 
on the retina at P, and will remain on that spot till it is effaced. 
If the eye A is noiv raised to B, the impression will still be at P 
in the elevated eye, having risen only /oth of an inch, or the 
height through which the eye has been raised by pressure. This 
small space is not very visible to an ordinary observer,” &c. 
How could Dr Brewster conceive, that we could push the 
eye-ball aside, without its revolving on its centre ? What we 
have above all to admire in the position of the eye-ball, is the 
manner in which it is poised, and prepared to revolve on the 
slightest action of its muscles, or lateral pressure. But we must 
prove that it actually does revolve in this very experiment. 
It will be observed, that, in this diagram of Dr Brewster’s, 
the line of the axis of the eye, when the organ is pushed aside, 
