the crystalline humour, which is a double-con- 
vex lens or great refracting power, and the rest 
of the eye is tilled with a jelly-like substance 
called the vitreous humour. The iris, which 
is the coloured part of the eye, is an opaque 
membrane which is perforated by a small 
hole, the pupil, through which the rays of 
light must pass to the crystalline humour. 
The optic nerve enters at the under part, 
and is spread all over the interior surface, at 
the back of the eye, in the form of a line net- 
work, and therefore is called the retina. The 
student of optics will see from this, that the 
eye is altogether calculated to act as a convex 
lens of strong refractive powers. 
It has already been explained, that from 
every luminous point of a visible object, 
cones or pencils ot' light are emitted or re- 
flected in every direction ; but to produce 
vision, it is necessary that they should be con- 
centrated or converged to such a point as to 
make a forcible impression on the retina. 
Thus from the luminous body A, Plate II. 
(tig. 22.) the rays r, r, r, are sent in various 
directions. Those which fail upon the trans- 
parent cornea CC, are there refracted in 
such a manner as to enter the pupil atp, and 
in passing the chrystalline lens or humour they 
suffer a second refraction, and are converged 
to a point or focus at the point a on the re- 
tina. Now it is evident, that if the rays 
could have passed the humours of the eye in 
their natural direction, that is, in the direction 
of the cone or pyramid CAC, they would 
have made upon the retina a very extensive 
but feeble impression, such as we know by 
experience could not produce distinct vision ; 
to obviate this it is appointed by the all-wise 
Author of our existence, that by the force of 
the refraction which they suffer in the eye, 
they should form another cone opposed to 
the first at its base, and the apex o( which is 
at a, and thus an impression sufficiently forci- 
ble to produce distinct vision is made on the 
retina. 
In the preceding instance, the luminous 
body A was considered as a point; and what 
has been said of it will apply to every point 
of a visible object, which is capable of trans- 
mitting or reflecting to the eye a pencil or 
collection of rays. Thus we may easily sup- 
pose that from every part of the arrow O A 
B, (fig. 23.) cones or pencils of light may be 
transmitted ; these, like all pencils, or collec- 
tions of rays, coming from a point, will di- 
verge, and will fall upon the eye in some de- 
gree divergent, or in the form of cones or py- 
ramids. 
The pencil of rays OEIF will then paint 
the extremity O in the point I ; the pencil 
BFME will aiso paint the extremity B in the 
point M; and since all the points between O 
and B are represented between I and M, of 
course IM will be the image of OB. Hence 
it is evident, that by means of this refraction 
there are certain points at which the rays of 
light, after passing the pupil, cross each other, 
and the image which is formed on the retina 
is consequently inverted. 
Artificial eves are sold by the opticians, in 
which all the humours are made of different 
kinds of glass, and may be separated at plea- 
sure. At the back part, where the retina is 
supposed in the natural eve to receive the 
converged rays, is placed a piece of ground 
glass, where the image from the opposed ob- 
OPTICS. 
ject is rendered in an inverted position, as in 1 
a camera obscura. The same effect may be 
produced with a natural eye, and the nature 
of vision may be thus experimentally demon- 
strated: if a bullock’s eye is taken fresh,' the 
posterior coats dexterously removed even to 
the vitreous humour, and if a piece of white 
paper is then placed at the part, the image 
ot any bright object which is placed before 
the eye will be seen distinctly painted on the 
paper, but in an inverted position. 
If the humours of the eye, through age or 
weakness, have shrunk or decayed, the cor- 
nea will then be too flat; and the rays, not be- 
ing sufficiently bent or refracted, arrive at the 
retina before they are united in a focus, and 
would meet, if not intercepted, in some place 
behind it, as in Plate II. fig. 25. They 
therefore do not make an impression suffici- 
ently correct and forcible, but form an indis- 
tinct picture on the bottom of the eve, and 
exhibit the object in a confused and imperfect 
manner. This defect of the eye is therefore 
remedied by a double-convex lens, such as 
the common spectacle-glasses, which, bv 
causing the rays to converge sooner than they 
otherwise would, afford that aid to this defect 
of nature which the circumstances of the case 
may require ; the convexity of the glass being 
always proportioned, by one who is capable 
of directing in the choice of spectacles, to the 
deficiency in vision. 
If, on the contrary, the cornea is too con- 
vex, the pencils of rays will unite in their 
foci before their arrival at the retina, as in 
fig. 26, and the image will also be indistinct. 
This defect is remedied by concave glasses, 
which cause the rays to diverge ; and conse- 
quently, by being properly adapted to the 
case, will enable the eye to form the image 
in its proper place. 
The rays of light being emitted or reflected 
from a visible object in all directions, it must 
be plain that some of them from every part 
of it must reach the eye. Thus the object 
AB (Plate II. fig. 28) is visible to an eye in 
any part where the rays A a, A b, Ac, Ad, 
Ac, Ba, B b, Be, B d, Be, C a, C b, Ce, Cd, 
and Ce, can come. But though rays are re- 
flected from eyery point of the object to 
every part of the circumambient space, yet 
it is evident that only those rays which pass 
through the pupil of tiie eye can affect the 
sense; and those rays also give the ideas. of 
colour, according to the properties of those 
bodies which transmit or reflect them. 
As the d irection in which the extreme pencils 
of light cross each other in the eye, bears a 
due proportion to the angle in which they are 
transmitted from the object to the eye, it is 
evident that the image formed upon the retina 
will be proportioned to the apparent magni- 
tude ; and thus we have our first ideas of the 
size and distance of bodies, which, however, 
in many cases are corrected by experience. 
The nearer any object is to the eye, the larger 
is the angle by which it will appear in the 
eye, and therefore the greater will be the 
seeming magnitude of that body. In Plate 
II. fig. 24, let AB be an object viewed di- 
rectly by the eye QR. From, each extre- 
mity draw the lines AN and BM, intersect- 
ing each other In the crystalline humour at I. 
Then draw the line IK in the direction in 
which the eye is supposed to look at the ob- 
ject- Tins angle All) is then the- optical or 
303 ; 
visual angle ; and the line IK is called the op- 
tical axis, because it is the axis of the lens or 
crystalline humour continued to the object. 
'Phe apparent magnitude of objects, then, 
depending thus on the angle under which 
they are seen, will evidently vary according 
to their distances. Thus different objects, as 
AB, CD, EE, the real magnitudes of which 
are very unequal, may be situated at such 
distances from the eye as to have their appa- 
rent magnitudes all equal ; for if they are si- 
tuated at such distances that the rays AN,. 
BM, shall touch the extremities of each, they 
will then appear all under the same optical 
angle, and the diameter MN of each image 
on the retina will consequently be equal. 
In the same manner object's of equal mag- 
nitude, situated at unequal distances, will ap- 
pear unequal. For let AB and GII, two 
objects of equal size, be placed before the 
eye at different distances, IK and IS; draw 
the lines CP and HO, crossing each other in 
I ; then OB, the image formed by the object 
GH on the retina, is evidently of a greater 
diameter than the image MN, which repre- 
sents the object AB; in other words, the ob- 
ject GH will appear as large as an object of 
the diameter TV, situated at the same place 1 
as the object AB. 
To render the subject still clearer, suppose 
the object P1K (see Blate II.. fig. 27) to be at. 
a hundred yards distance, it will form an an- 
gle in the eye at A. At two hundred yards 
distance the angle it makes will be twice as 
■small in the eye at B. Thus to whatever mo- 
derate distance the object is removed, the 
angle it forms in the eye will be proportion- 
ably less, and therefore the object will be di- 
minished in the same proportion. 
Hence it follows, that objects situated at 
different distances, whose apparent magni- 
tudes are equal, are to each other as their' 
distances from the eve ; and by the same 
rule, equal objects situated directly before- 
the eye, have their apparent magnitudes in a 
reciprocal proportion to their distances. 
T his last proposition must, however, be re- 
ceived with some allowance ; for it is only 
applicable to very distant objects, and to 
those where the sense is notcorreeted by the 
judgment. For if the objects are near, we 
do not judge of their magnitude according to 
the visual angle. Thus, if a man of six feet 
high is seen at the distance of six feet under 
the very same angle as a dwarf of only two 
feet high at the distance of tw-o feet, still the 
dwarf will not appear as large as the man, be- 
cause the sense is corrected by the judg- 
ment. 
In most cases, however, where the dis- 
tance is considerable, the rule will be found 
accurate ; and as it lias its foundation in na- 
ture, most of the phenomena of vision will 
be explained by having recourse to the prin- 
ciples here laid down. If the eye is placed 
above a horizontal plain, the different parts 
of this plain will appear elevated in propor- 
tion to their distance, till at length they r will 
appear upon a level with it. For in propor- 
tion as the different parts are more distant) 
the rays which proceed from them form an- 
gles with the optical axis IK (Plate 11. fig; 
24) more and more acute, and at length be- 
come almost parallel. This is the reason 
why, if we stand on the sea-shore, those 
parts o£ the ocean which^ are at a great diy- 
