5b2 OPT 
than the pofterior, and rounded off at the edges, E, E, as 
the figure reprefents. This humour, which is nearly of 
the confiliency of hard jelly, decreaflng gradually in den- 
lity from the centre to the circumference, is called the 
erufuilline humour. It is kept in its place by a mufcle, 
called the Ugammtvm ciliare, which takes its rife from 
the junftion of.the choroides and cornea, and is a little 
convex towards the uvea. The anterior furface of this 
mufcle, and the pofterior furface of the uvea, are covered 
■with a black mucus, evidently defigned to abforb any of 
the extreme rays which may happen to reach fo far, and 
which might be reflected to the retina, and produce con- ' 
fuiion in the vifion. 
The cavity of the eye, between the cornea and the cryf¬ 
talline humour, is filled with a tranfparent fluid, like 
water, called the aqueous humour. The cavity between 
the cryftalline humour and the back part of the eye, is 
alfo filled with a tranfparent fluid, rather more vilcous 
than the former, called the vitreous humour. 
It is not eafy to afeertain, with great accuracy, the re¬ 
fracting powers of the feveral humours; the refrafting 
powers of the aqueous and vitreous humours are nearly 
equal to that of water; the ref rafting power of the cryf¬ 
talline humour is fomewhat greater, as is manifeft from 
its figure, and from the circumftance that perfons couched, 
(in which cafe the cryftalline lens is taken out) are obliged 
to ufe convex glades. 
The furfaces of the feveral humours of the eye are fo 
fltuated as to have one line perpendicular to them all. 
This line ADF is called the axis of the eye, or the optic 
axis. The dimenfions of the feveral parts of the eye may 
be feen in Harris’s Optics, page 94. 
The point in the axis, at which the objeft and the image 
upon the retina fubtend equal angles, is not far diftant 
from the pofterior furface of the cryftalline lens, though 
its fituation is lubjeft to a fmall change, as the figure of 
the eye or the diftance of the objeft is changed. See 
Harris, p. 97. 
From the confideration of the ftrufture of the eye, we 
may eafily underftand how notices of external objefts are 
conveyed to the brain. Thus, let PQR (fig. 2) be an objeft: 
towards which the axis of the eye is directed; then, the 
rays which diverge from any point Q, and fall upon the 
convex furface of the aqueous humour, have a degree of 
convergency given them ; they are then refrafted by a 
double-convex lens, denfer than the ambient mediums, 
which increafes the convergency ; and, if the extreme 
rays QH, QI, have a proper degree of divergency before 
incidence, the pencil will be again collefted upon the re¬ 
tina at q, and there form an image of Q. In the fame 
manner, the rays which diverge from any other points, 
P, R, in the objeft, will be collefted at the correfponding 
points j), r, of the retina, and a complete image, pqr, of 
the objedt PQR, will be formed there. The impreilion, 
thus made, is conveyed to the brain by the optic nerve, 
which originates there, and is evidently calculated to an- 
fwer this purpofe. 
Since the axes of the feveral pencils crofs each other 
within the eye, the image upon the retina is inverted with 
relpeft to the objeft ; and if, by any means, the image of 
an ereft objeft be ereft upon the retina, that objeft ap¬ 
pears inverted. If the outer coat be taken from an ox’s 
eye, whilft it is warm, the images of external objefts are 
obferved to be inverted upon the retina. 
It has been objefted, that, if the images upon the retina 
he inverted, external objefts ought to appear inverted. 
To which it may beanfwered, that experience alone teaches 
us what fituation of the external objeft correfponds to a 
particular impreflion upon the retina; nor is it of any con- 
fequence what that impreflion is, or in what manner it is 
made ; but, whenever the fame effedf is produced upon the 
organ, we expeft to find the fame external objeft, and in 
the fituation to which our former experience direfts us. 
Thus, if the point P move along the line PQR from the 
right to the left, the image p moves from the left to the 
I c s. 
right upon the retina: and, in general, whenever the 
image upon the retina moves from the left to the rigtr, 
we are led, by experience, to conclude that the objeft really 
moves from the right to the left. 
If the form of the eye, the fituation of the feveral hu¬ 
mours, and their refpeftive furfaces, remain unaltered, it 
is manifeft that thofe rays only, which diverge from points 
at a particular diftance, can be colledled upon the retina. 
Thus, if the image of Q, fig. 3. be formed exactly upon 
the retina, the image of S, a point farther from the eye 
than Q, will be formed within the eye; therefore, the 
rays which proceed from this point will be diffuled over 
fome fpace upon the retina ; and, if they are mixed with 
the rays which diverge from other points in the objeft, 
necelfary to be diftinguilhed from the former, the vifion 
will be indiftinft. Again, the rays which diverge from 
T, (fig. 4.) a point nearer to the eye than Q, will, after re- 
fraftion, converge to t, a point behind the retina; in this 
cafe alfo, they will be diffufed over lome fpace upon the 
retina, and the vifion, as before, will be indiftinft. 
By what change in the conformation of the eye we are 
enabled to lee objefts diftinftly at different diftances, is 
not fully afeert'ained. The faft itfelfis fufficiently mani- 
feft; but authors differ in opinion as to the manner in 
which the effeft is produced. It is fuppofed by fome, that 
the general figure of the eye is altered ; that, when the 
objeft to be viewed is near, the length of the eye, mea- 
fured along the axis, is increafed by thelateral preffure of 
external nuifcles ; and, on the contrary, when the objeft 
is remote, that the length of the eye is diminilhed by the 
relaxation of that prelfure.. Others fuppofe the effeft to 
be produced by a change in the place or figure of the cryf¬ 
talline humour. Others, by an alteration in the diameter 
of the pupil. Others aferibe the effeft to a change in the 
curvature of the cornea. Much ftrefs cannot be laid upon 
the firft of thefe caufes, as diftinguilhed from the laft, fince 
its exiftence is not proved by experiment; and there is 
no neceflity for recurring to a bare hypothefis of this 
kind. With refpeft to thefecond, the ligamentunr ciliare 
does not appear lufficiently ltrong to produce any confi- 
derable change in the form or fituation of the cryftalline 
humour. And as it is clearly afcertained, (Phil. Tranf. 
vol. Ixxxv. p. 6.) that perfons couched can lee diftinftly 
at different diftances, we mult conclude that the effeft is 
not to be aferibed to any change in this humour. A 
change in the aperture of the pupil has very iittie effeft 
in rendering objefts diftinft at different diftances. 
It has been demonftrated, that, if the polition of an ob¬ 
jeft, placed before a lens, be given, the image is formed 
at the fame diftance, whether the rays proceeding from 
the objeft pafs through a greater or fmaller portion of the 
lens. The lame is manifeltly true of the eye, fo long as 
the feveral mediums of which it coniifts retain their forms 
and pofitions; the expanlion, therefore, or contraftion of 
the iris cannotcaufe the images, of objefts at different dif¬ 
tances from the eye, to be formed on the retina. If, how¬ 
ever, lateral rays are Hopped by the iris, the indiftinftnefs, 
ariftng from the diffulion of the rays in each pencil over 
fome fpace on the retina, will be lelfened : but this can 
only take place in a fmall degree, and when the objefts are 
very near to the eye. It is on this account that a fmall 
hole in a thin plate enables us to view objefts at a lefs dif¬ 
tance than we could with the naked eye, as it anfwers the 
purpofe of a farther contraftion of the pupil, and excludes 
thole rays in each pencil which diverge too much. This 
affiftance cannot be made ufe of to any great extent, be- 
caule the image upon the retina will loon become indif- 
tinft for want of light; and the infleftion of rays at the 
fides of the hole, will render it confuted. 
The principal change by which the effeft is produced, 
feems to be an alteration in the curvature of the cornea. In 
order to Ihow that luch a change takes place, Mr. Ramf- 
den fixed the head of a fpeftator fo fecurely, that no de¬ 
ception could arife from its motion, and direfted him to 
look at a diftant objeft; whilft the eye was in this fituation. 
