ANA" 
G-orpufcles. Thefe form a falirie pellucid liquor that may 
be evaporated, and never ceafes to be poured over the an¬ 
terior Jfurface of the eye ; but never runs over the cheeks, 
unlefs collected, by a foreign caufe, in larger than ufual 
quantity. This liquour is exhaled partly from the arteries 
of the conjunctiva, and partly proceeds from the lachrymal 
gland. The reparation of the tears is increafed by the 
more frequent contraction of the orbicular nuifcle, either 
from irritation, or fome forrowful pailion; by which means 
the tears are urged over, and vvaflt the whole furface of 
the eye and conjunctiva. 
Among the coats of the eye, the iris is the only one 
po(Tefling motion. Though it has little fenfation, and is 
not endowed with any mechanical irritability; yet, in a li¬ 
ving man, quadruped, or bird, it is contracted on every 
greater degree of light, and is dilated on every fmaller 
one; hence it is rendered broader for viewing diftant ob¬ 
jects, and narrower for viewing fuch as are near. The 
caufe of this dilatation feenis to be aremifiion of the pow¬ 
ers-refilling the aqueous humour; an argument of which 
is the dilatation of the pupil, occafioned by debility, or 
fucceeding fyncope and death. 
Before we can attain any fatisfaCtory knowledge of the 
properties of the eye, it will be necelTary to fay fomething 
of the nature of light. Light is an extremely fubtile ft n- 
id, penetrating with facility bodies of the clofelt texture, 
and greatelt denfity, in a reCtilineal direction, with the ama¬ 
zing velocity of ten millions of miles in a minute. Every 
luminous body has the peculiar property of lending forth 
rays of light in every poflible direction, which, falling on 
the furfaces of'furrounding cbjeCts, are reflected thence 
to our eyes. Rays of light falling perpendicularly on the 
furface of a tranfparent body, pal's through 'lie body 
without changing their direction ; but rays falling oblique¬ 
ly on the furface pafs through the body with a change of 
their direction nearer to the perpendicular; and this change 
of direction is called refraElion. 
The rays of light, whether direCt or infleCted, fall upon 
the tunica cornea of the eye, fo as to form a very fharp 
cone between the i ucid point and the membrane upon which 
they are fpread; the balls of which cone will be the furface 
of the cornea, and the apex the radiant point; yet fo that 
all rays may, without any fenfible error, be reckoned pa¬ 
rallel with each other. Among thefe, there are fome rays 
reflected back from the cornea, without ever penetrating 
the furface; namely, all fitch as fall upon that membrane 
in a greater angle than that of forty degrees. Others, 
which enter the cornea at very large angles, but lefs than 
the former, and far! in betwixt the iris and the titles of the 
cryltalline lens, are fuffocated or loft in the black paint 
that lines the iris and the ciliary procelfes ; but thofe rays 
only fall upon the furface of the lens which enter the cor¬ 
nea at fmall angles, not much diftant from the perpendi¬ 
cular, or, at moft, not exceeding twenty-eight degrees. 
By this means, all thofe rays are excluded which the re- 
fraCting power of the humours of the eye could not be 
able to concentrate or bring together upon the retina; 
without which they would paint the objeCt too large and 
tonfufedly. The rays falling on the cornea are therefore 
refraCted, and pafs through the aqueous humour in a more 
parallel, or perhaps converging, direction, by which con¬ 
trivance a greater number of rays fall on the cryltalline 
than if they had not previoufly pafifed through the cornea 
and the aqueous humour. But fince the necelTary offices 
of human life require a diltinCt objeCt to be painted upon 
the retina, not only by the rays which come from one cer¬ 
tain diltance, but likevvife by rays which come from very 
different par's more or lefs diftant; it has therefore been 
thought that a necelTary change, produced by its own 
caufes, is made on the eye. Others have fuppofed the 
lens moveable by the powers before-mentioned. This 
art of feeing diftinCtly at different diltances is faid to be 
learned by experience, it being unknown to thofe who 
have been lately couched. It is alfo faid, that, in an ar¬ 
tificial eye, the life and neceffity of this motion may be 
O M Y. S99 
plainly perceived. The point of dilcin6c vifion is in that 
part of the retina where the given objeCt is painted in the 
leai’r compafs poliible. The powers cauffng the vilual rays 
to unite on the retina, are often very different in the two 
eyes of the fame perfon, the one being long-lighted and 
the other Ihort-fighted. 
Thefe, and other commonly received opinions, are taught 
by mathematicians, who more readily perceive the neceffity 
of thefe changes. But yet there is no power in the human 
eye which can either move the Cryffaliine humour out of 
its piace, or comprefs it. Befides, we do not perceive this 
faculty in ourfelves; for we move a book nearer to our 
eyes when it is too far off, fo as to appear confufed, which 
we fliould have no occallon to do, if by changing the in¬ 
ternal figure of the eye we could correct the fault of the 
diltance. And, through a fmall hole, we perceive an ob¬ 
jeCt fingle only in the point of diltinCt vifion, but double 
in every other. Perhaps the contraction of the pupil may 
enable to fee near objeCts more diftinCtly v This contrac¬ 
tion, however, is not in all people fufficient for the pur- 
pofe. There are feveral people, efpecially fuch as lead a 
fedentary life, and fuch as are employed in examining mi¬ 
nute objeCts, whofe cornea is too convex and denfe, whofe 
cryltalline lens is too gibbous and folid, and whofe eye is 
lengthened by the incumbent weight of the humours, and 
perhaps the humours themfelves are too denfe; and in the 
fame perfon the gye may probably have all thefe defeCts 
joined together. People labouring under one or more of 
thefe inconveniences have an iris that is fenfible in a very 
fmall degree of light, which circumftance makes them 
twinkle with the eye-lids when they are in a Itrong light, 
and they are called myopes or Ihort-fighted. In thefe, the 
point of diltinCt vifion is very near to the eye, commonly 
from one to feven inches from the cornea; but they fee 
remoter objeCts more obfcurely, without being able todil- 
tinguilh their parts. The reafon of this is evident; fince, 
from the fore-mentioned caufes, there is’ a greater re fra Cl¬ 
ing power of the humours, by which the diftant, and con- 
feqitently parallel, rays are obliged to meet in their focus 
before fhe retina, from whence fpreading again, they fall 
upon the retina in many points. The remedy for this fault 
in the fight is to correct it in its beginning, by looking at 
diftant rather than near and minute objeCts; by the ufe 
of concave glades, or.by viewing things through a fmall 
hole, by which the light is weakened. When thediforder 
is confirmed, the remedy is a concave lens, which takes off 
a degree of the refracting power in the humours, cornea, 
and cryffaliine lens, in proportion as it is more concave; 
by which means the focus of rays from remote objeCts is 
removed farther behind the cornea, fo as to fall upon the 
retina. 
The other diforder of the fight, contrary to the former, 
troubles people who often look at very diftant objeCts, ancf 
is more efpecially familiar and incurable in old people. 
In fuch, the cornea and cryffaliine lens are flatter, and the 
humours of the eye have a lefs refraCting power. Hence 
near objeCts, whofe rays fall very diverging upon the cor¬ 
nea, appear confuted ; becaufe the converging or refract¬ 
ing powers of the eye are not fufficient to bring the rays 
together in a focus upon the retina; but the rays go on 
fcattered beyond the retina, and throw the point of their 
pencil behind the eye, from whence vifion is confufed. 
The point of diftinCt vifion among prijbyopi, or old or long- 
fighted people, is from the diftance of fifteen incites to 
three feet. Such perfons are relieved by looking through 
a black tube held before the eye; by tlte ufe of which the 
retina grows tenderer, and *he rays come to the eye in a 
parallel direction. The remedy here is a convex lens, 
which caufes the rays to converge and unite together foon- 
er in a focus, that it may not fall behind the eye, but upon 
the retina. The medium between the ffiort and long light¬ 
ed eye is the belt, by which a perfon can fee diitinCtly 
enough objeCts that are both near and remote ; and of this 
kind we reckon an eye that is able to read diitinCtly at the 
diftance of one foot. But a good eye requires other ne- 
ceflary 
