375 
Mr Charles Bell on the Motions of the Eye. 
its outline; and, jnoving from point to point, the figure is as- 
certained by the same operation by which we have been in- 
formed of its distance’ from us, and its relation to other objects. 
Our motions to and from an object, and the movements of the 
hand and the head, all aid in perfecting the vision. But, in the . 
papers which Dr Brewster has criticised, I was treating of the 
nerves of the orbit, and I had occasion to speak only of the 
muscles of the eye. 
teut to return to the ofiices of the eye. This continual actk 
vity and searching motion of the eyeball, is owing to a well 
known property in the retina, — the more exquisite sensibility in 
that part of it which is directly in the axis of vision. When we 
look upon a scene or upon a wall, we see only a small part of it 
perfectly, whilst the objects around are presented to the eye in 
comparative obscurity. But, with a motion rapid as thought^ 
the centre of the retina is opposed to all the objects in the field 
successively ; and with a glance, of which, in one sense, we are 
scarcely conscious, the whole appears equally vivid. 
If, however, by an effort, we try to fix the eye upon a spot, 
we shall be aware that the surrounding objects are imperfectly 
seen ; and, at the same time, we shall be made sensible of the rest- 
lessness of the eye to search out whatever is obscure. It is this 
propensity to penetrate what is obscure, that makes the eye-ball 
roll continually, when, from a morbid state of the retina, objects 
in one part of the field of vision appear covered with a cloud. 
There is an uncontrollable effort of the eye to present its axis 
towards the objects thus obscured ; and it is in this manner that 
we continually follow what appears a cloud, and which, by the 
motion of the eye, as necessarily flies from us. 
To controvert my opinion. Dr Brewster has placed himself 
upon a stool which revolves; and is at the pains to have a 
leathern belt, and a friend to turn him round. I am truly at a 
loss to know whether the Doctor has placed himself here in per- 
fect philosophical simplicity and singleness of heart. However 
that may be, thus he concludes : That the notion of place or 
relation depends on the muscles of the assistant’s arm, conveyed 
by some sympathetic action to the observer’s eye, along the 
leathern belt,— a result so inadmissible,” &c. 
Is it possible that Dr Brewster, with the advantage of the 
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