VISION. 
When any point of an object is seen dis- 
tinctly with both eyes, the axis of both 
eyes are directed to that point, and meet 
there ; and then the object appears single, 
though looked at with both eyes ; for the 
optic nerves are so framed, that the corres- 
pondent parts in both eyes lead to the same 
place in the brain, and give but one sensa- 
tion, and the image will be twice as bright 
with both eyes as with one. But if the axis 
of both eyes be not directed to the object, 
that object will appear double, as the pic- 
tures in the two eyes do not fall upon cor- 
respoudent or similar parts of the retina. 
The best eye can hardly distinguish any 
object that subtends at the eye an angle 
less than half a minute, and veiy few can 
distinguish it when it subtends a minute. 
If tlie distance of two stars in the heavens 
be not greater than this, they will appear as 
one. Though men may see distinctly at 
different distances, by altering the position 
and figure of the crystalline, yet they can 
only see distinctly within certain limits, and 
nearer than that, objects appear confused. 
But these limits are not the same in differ- 
ent people. A good eye can see distinctly 
when the rays fall parallel upon it, and 
then the principal focus is at the bottom 
of the eye; a man can judge at a small 
distance, with a single eye, by frequently 
observing how much variation is made 
in the eye to make the object distinct, 
and from this a habit of judging is ac- 
quired. But this cannot be done at 
great distances, because, though the dis- 
tance be varied, the change in the eye 
becomes then insensible. But a man 
can judge of greater distances with both 
eyes, than he can with one. For the eyes 
being at a distance from one another, as 
long as that distance has a sensible propor- 
tion to the distance of the object, one gets 
a habit of judging, by the position of the 
axis of the eyes, which are always directed 
to that point. For different distances re- 
quire different positions of the axis; which 
depend on the motions of the eyes, which 
we feel. But in very great distances no 
judgment can be made from the motion of 
the eyes, or their internal parts. There- 
fore we can only guess at the distances 
from the magnitude, colour, and the posi- 
tion of inteijacent bodies. Dimness of 
sight generally attends old people, and this 
may arise from two causes. 1. By the eyes 
growing flat, and not uniting the rays at the 
retina, which causes indistinctness of vision ; 
or, 2. By the opacity of the humours of the 
eye, whicli in time lose their transparency, 
in some degree ; from whence it follow's 
that a great deal of the light that enters 
the eye, is stopped and lost; and every 
object appears faint and dim. Hence the 
necessity of spectacles. 
If objects are seen through a perfectly 
flat glass, the rays of light pass through it 
from them to the eye, in a straight direc- 
tion, and parallel to each other, and conse- 
quently the objects appear very little either 
diminished or enlarged, or nearer, or fln- 
ther off, than to the naked eye ; but if tlie 
glass they are seen through have any degree, 
of convexity, the rays of light are directed 
from the circumference towards the centre, 
in an angle proportional to the convexity of 
the glass, and meet in a point, at a greater 
or lesser distance from the glass, as it is more 
or less convex. This point, where the rays 
meet, is called the focus, and this focus 
is nearer or further off, according to the 
convexity of the glass ; for as a little con- 
vexity throws it to a considerable distance, 
so when the convexity is much, the focus is 
very near. Its magnifying power is also in 
the same proportion to the convexity: for 
as a flat glass scarcely magnifies at all, the 
less a glass departs from flatness, the less 
of Course it magnifies ; and the more it ap- 
proaches towards the globular figure, the 
nearer its focus is, and the more its magni- 
fying power. People’s different length of 
sight depends on the same principle, and 
arises from more or less convexity of the 
cornea and crystalline humour of the eye ; 
the rounder these are, the nearer will thd 
focus or point of meeting rays be, and the 
nearer an object must be brought to see it 
well. The case of short-sighted people is 
only an over-roundness of the eye, which 
makes a very near focus ; and that of old 
people is a sinking or flattening of the eye, 
whereby the focus is thrown to a great dis- 
tance, so that tlie former may properly be 
called eyes of too short, and the latter eyes 
of too long a focus. Hence too, the remedy 
for the last is a convex glass, to supply the 
want of convexity in the eye itself, and 
brings the rays to a shorter focus ; whereas 
a concave glass is needful for the first to 
scatter the rays and prevent their coming 
to a point too soon. The nearer any ob- 
ject can be brought to the eye, the larger 
will be the angle under which it appears, 
and the more it will be magnified. Now, 
that distance from the naked eye, where 
the generality of people are supposed to see 
small objects best, is about six inches, con* 
