ADJUSTMENT OF THE EYE FOR VARYING DISTANCES. 427 
has served as a guide to Art; or, in other words, the Divine 
Artificer has thus condescended to teach the human workman. 
_ 550. There is another wonderful arrangement in the 
healthy Eye, which the optician can only imitate in his 
instruments in a very bungling manner. It is that by which 
the eye adapts itself to view objects at different distances 
from it, with equal distinctness. If we look at a near object 
with a Telescope, adjusting the instrument so as to see it dis- 
tinctly, and then turn it towards a remote object, we shall 
not see the latter with equal clearness until the instrument 
has been again adjusted. If we then turn it back to the 
nearer object, we shall find that the change in the adjustment 
occasions the representation of it to be now indistinct; and 
in order to bring back the image to its former clearness, it is 
Fequisite to re-adjust the instrument to its first condition. 
This is a necessary consequence of the optical law, that the 
distance of the image from the lens which forms it, varies 
With that of the object,—being increased as the object is 
brought nearer, and diminished as it recedes. If the Eye 
were constructed in the same manner, we should not be eo 
to see distinctly, without the aid of artificial assistance, at any 
Other distance than that for which it is adjusted. Hence if a 
perfect picture of an object situated at twelve inches’ distance 
from the eye, were formed upon the retina, we should not be 
able to see it clearly when brought to the distance of six inches, 
mor when removed to the distance of six feet ; because in the 
first of these cases the rays would not be brought to a focus 
upon the retina, but at a point behind it (if they were 
owed to pass on unchecked); whilst in the second, they 
would be brought to a focus at a point nearer than the retina, 
ind would consequently begin to separate again before they 
‘each it. 
551. But the healthy eye possesses a power of perfect 
idjustment to the viewing of objects situated at different 
istances ; and this without any effort or intention on our 
arts but, as it were, by an instinctive operation. That such 
change really takes place, we may readily convince ourselves, 
dy looking at a near and at a distant object placed in the 
ame line,—a pencil-case, for instance, held up at a few inches 
from the eye, and a chimney half a mile off. We shall find 
hat no effort of attention will enable us to see them both 
