OPTICS. 
coats, which are formed by the expansion 
of the different component parts of the 
optic nerve, vi~. the sclei otrca, S S. The 
choroides, DD; and 3. The retina, TT. 
'I’lie sclerotica is outermost; it is very 
st rong, and the fore part, which is transpa- 
rent, and somewhat prominent, is called 
the cornea, C. The choroides is next in 
order, and has a circular perforation, P, 
called the pupil, immediately behind the 
middle of the cornea: the part II. ot the 
choroides, visible behind the cornea, is 
flat ; it is called the iris, or nvea, and is 
differently coloured in different persons. 
The retina is the inmost coat, it extends 
round the eye till it meets the ciliary liga- 
ments, Q Q, membranes proceeding from 
the choroides, and attached to the capsula 
or filament, which incloses the crystaline 
humour, R. The crystaline is the most * 
dense of the three humours, and is in the 
shape of a double convex lens, whose fore 
part has the less curvature ; the cavity be- 
tween the cornea and the crystaline is 
occupied by the aqueous humour, which 
has rather the least density of the three, and 
the space between the bottom of the eye 
and the crystaline is filled by the vitreous 
humour, V. 
Objects presented to the eye have their 
images painted on the back part of the re- 
tina, the rays of the incident pencils con- 
verging to their proper foci there by the 
refraction of the different humours ; and 
for this office they are admirably adapted; 
for as the distance between the back and 
front of the eye is very small, and the rays 
of each of the pencils that form the image 
fidl parallel, or else diverging on the eye, a 
strong refractive power is necessary for 
bringing them to their foci at the retina ; 
but each of the humours, by its peculiar 
form and density, contributes to cause a 
convergence of the rays ; the aqueous from 
its convex form; the crystaline by its 
rlouble convexity and greater density than 
t he aqueous ; and the vitreous by a less 
density than the crystaline joined to its 
concave form. 
These things are manifest from what has 
been already said. The structure of the 
eye is in general adapted to the reception 
of parallel rays ; but as the distances of 
visible objects are various, so the eye has 
powers of accommodating itself to rays 
proceeding from different distances, by al- 
tering the distance of the crystaline from 
the retina, which is done by the action of 
the ciliary ligaments. 
That this change of situation in the 
crystaline is adequate to such accommo- 
dation, may be thus shewn. Suppose a 
pencil of rays to diverge from a point. A, 
(fig. 18.) at a distance from the eye less 
than that which admits distinct vision in the 
usual situation of the humours : the rays 
would come to a focus, V, behind the re- 
tina, LM. Let the crystaline, OP, be 
brought forward, and, C V, the distance of 
the focus from the crystaline will be in- 
creased ; but, because of the great propor- 
tion that, A C, the smallest distance that 
admits distinct vision has to, F C, the focal 
length of the crystaline, the distance, 
C G, of the crystaline from the retina 
will be more increased than CV, so that 
C G and C V may become equal, and thus 
the focus made to fall exactly on the retina. 
These powers of accommodation are 
however limited, and the sight is said to be 
perfect when the eye can adapt itself to 
any distance within the usual limits, and 
when it cannot, visiqn is indistinct. 
Defective sight arises from an incapacity 
of altering the position of the crystaline 
within the usual limits. 1. When it cannot 
be brought close enough to the cornea, near 
objects appear indistinct ; to this defect 
people in years are generally subject. 2. 
Where the crystaline cannot be drawn suf- 
ficiently near to the retina, remote objects 
appear indistinct ; this is the defect under 
which myopes, or short-sighted people, 
labour. In each of these cases the images 
of the dilferent points in the object would 
be diffused over small circles on the l etina ; 
and so being intermixed and confounded 
with each other, would there form a very 
confused picture of the object : for in the 
former case (fig. 19), the image of any 
point would be formed behind the retina, 
as the refraction of the eye is not suffici- 
ently strong to bring the rays (diverging so 
much as they do in proceeding from a near 
point) to a focus at the retina. This detect 
will therefore be remedied by a convex 
glass, aft, which makes the point whence the 
rays now proceed more distant than the 
object; therefore the rays falling on the 
eye will now diverge less than before, or 
else be parallel, and will of course be 
brought to a nearer focus, viz. at the re- 
tina. 
In the latter case the image is formed 
before the retina (fig. 20) because the re- 
fractive power of the eye is too great to 
permit rays so little diverging (as they 
do in proceeding from a distant point) to 
