Dr. Hosack's Observations on Vision. 213 
in the different periods of life ? At the same time the original 
conformation of the eye, the diminution of its humours, and, 
probably, of the quantity of fat upon which the eye is lodged, 
are also to he taken into the account. But the external mus- 
cles becoming irregular and debilitated by old age, in common 
with every other muscle of the body, are not only incapable 
of compensating for these losses, but cannot even perform 
their wonted action, and thus necessarily have considerable 
influence in impairing vision. Again, does not the habit of 
long sight so remarkable in sailors and sportsmen, who are 
much accustomed to view objects at a great distance, and 
that of short sight, as of watchmakers, seal-cutters, &c. admit 
of an easy solution upon this principle? as we know of no 
part of the body so susceptible of an habitual action as the 
muscular fibre. 
Secondly. How are we to account for the weaker action of one 
•eye in the case of squinting? That this is the fact has been 
well ascertained ; Dr. Reid * upon this subject observes, that 
he has examined above twenty persons that squinted, and 
found in all of them a defect in the sight of one eye. Por- 
terfield and Jurin have made- the same observation. 
The distorted position of the eye has, I believe, been gene- 
rally attributed to the external muscles ; but no satisfactory 
reason has ever been given why the eye, directed towards an 
object, does not see it distinctly at the same distance as with 
the other. The state of the iris here cartnot explain it, as it 
contracts and dilates in common with the other ; nor can we 
suppose any muscles the lens might possess could have any 
* See his Inquiry into the Human Mind, page 322. 
