24 
On the late Mr. Hunter's preparation 
to my satisfaction, the many conclusions which are the result 
of such a power in this humour. 
The laws of optics are so well understood, and the know- 
ledge of the eye, when considered as an optical instrument, 
has been rendered so perfect, that I do not consider myself ca- 
pable of making any addition to it ; but still there is a power 
in the eye by which it can adapt itself to different distances 
far too extensive for the simple mechanism of the parts to ef- 
fect. This power writers upon this subject have been at great 
pains to investigate and explain. The motion of the crystal- 
line humour forwards and backwards, was asserted by some to 
be the cause ; while others supposed in the eye a power to al- 
ter its shape, so as to shorten or lengthen its axis, which 
altered the distance between the crystalline humour and the 
point of impression ; but we should consider that a part of the 
eye is itself a refractor, and that if its shape be altered so as to 
remove the crystalline humour from the point of impression, in 
order to enable it to bring a distant object to its proper focus 
on the retina, this effect will be in some degree counteracted 
by the anterior part of the eye refracting more than before, by 
being rendered more convex. But we have, in fact, no power 
capable of producing this effect ; for the straight muscles, so 
far from appearing to have this power, have been even sup- 
posed to flatten the eye, and shorten its axis : and it is very 
possible that the action of these muscles is such as tends to 
both effects ; but being in opposition to each other, the eye re- 
tains its shape, the insertion of these muscles being much 
more forwards than appears to be necessary for the simple 
motions of the eye. Further, when we consider that in many 
.animals the shape of the eye is unalterable, as in all of the 
