Dr . Hosack's Observations on Vision . 203 
peculiarly adapted. But is it inconsistent with the general 
laws of nature, or even with the animal ceconomy, that from 
their combination they should have a different action, and 
thus an additional use ? To illustrate this we need only wit- 
ness the action of almost any set of muscles in the body ; 
for example, in lifting a weight, the combined action of the 
muscles of the arm, shoulder, and chest, is different from the 
individual action of either set, or of any individual muscle ; or 
an instance nearer our purpose may be adduced, viz the ac- 
tions of the muscles of the chest and belly, making a com- 
pression upon the viscera, as in the discharge of urine, foeces, 
&c. But to question this fact would be to question the influ- 
ence of the will in any one of the almost infinite variety of 
motions in the human body. 
I presume, therefore, it will be admitted that We have the 
same power over these muscles of the eye as of others, and I 
believe we are no less sensible of their combined action ; for 
example, after viewing an object at the distance of half a mile, if 
we direct our attention to an object but ten feet distance, every 
person must be sensible of some exertion ; and if our atten- 
tion be continued but for a short time, a degree of uneasi- 
ness and even pain in the ball of the eye is experienced ; 
if again we view an object within the focal distance, /. within 
six or seven inches, such is the intensity of the pain that the 
exertion can be continued but a very short time, and we again 
relieve it by looking at the more distant objects ; this, I be- 
lieve, must be the experience of every person, whose eyes are 
m the natural and healthy state, and accordingly has been 
observed by almost every writer upon optics. 
But the power of this combination, even from analogy, 
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