on Muscular Motion. 
7 
exertion of this kind that he had first felt the complaint. It was 
clear from this account that this particular muscle had been 
strained, and was rendered unfit to bear any violent action. 
These cases will be sufficient to explain that a muscle, or set 
of muscles, may be unable to perform those actions which re- 
quire the greatest exertion, although capable of performing all 
the others. 
If then we consider the disease which causes the inability to 
see near objects as a strain upon the muscles, and compare it 
with the same disease in other muscles, there will be no diffi- 
culty in accounting for the bad effects produced by every thing 
that irritates, or weakens the parts themselves, or the general 
habit : it will follow, that such a mode of practice should be 
laid aside, and those means adopted by which the parts can be 
soothed in their sensations, and quieted and strengthened in 
their actions, since in that way only the muscular fibres can 
possibly, recover their tone. 
Of double Vision. 
Many opinions have been advanced to account for the single 
appearance of objects when seen by both eyes. 
Dr. Reid of Glasgow, who has taken much pains on this 
subject, has treated it with ingenuity and a great deal of know- 
ledge ; and the opinion he has advanced, of objects appearing 
single when the impressions from the object are made upon 
parts of the retina of the two eyes which correspond with each 
other, and double whenever that is not the case, is very strongly 
confirmed by the following observations upon double vision. 
There are two circumstances under which double vision 
