on Muscular Motion. 
21 
the ground, by means of elastic ligaments ; but they are brought 
down for use, which is an action not so often required, by 
muscles. 
In the adjustment of the eye it is the same; the state fitted 
for parallel rays is the effect of elasticity, but that for nearer 
distances, which is less frequently wanted, is the effect of 
muscular action. 
In these different instances, the intention is uniformly to 
avoid the expence of muscular action whenever the effect can 
be produced in any other way, as muscular actions consume 
a considerable quantity of blood, which is the nourishment of 
the body. 
That the adjusting the eye to near distances is the effect 
of an action, or exertion, was very evident to every gentleman 
concerned in these experiments. In changing the focus of our 
eyes, we were much astonished, particularly Sir Henry En- 
glefield, at the exertion required to adjust the eye to the 
near distances, and the facility with which it was adapted to 
distant ones ; the first was a strain upon the eye, the second 
appeared a relief to it. 
When the eye was intent upon the near object, it required 
the attention to be constantly kept up, or the object became 
indistinct ; and if we looked at it beyond a certain time, the 
eye was so much fatigued as to lose it at intervals. This cor- 
responds with other muscular actions, for whenever muscles 
are kept long in one state they begin to vibrate involuntarily. 
These circumstances explain what may be called a coup 
d’ceil, or the distinctness with which an object is seen when 
the eye is first fixed upon it. This arises from the nice ad- 
justment produced by the muscles when first thrown into 
