Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

b •£*
44 ^
, -vi::ir!euae; D.fcription. This difeafe flimvs itfelf by an uncom-
' 111011 contraction of the raafcles of the eye j whereby
the axis of the pupil is drawn towards the nofe, temples,
forehead, or cheeks, fo that the ptrfon cannot behold an
object diredtly.
Caufts, Prognq/is, &c. I. This difeafe may proceed
from cultorn and habit j while in the eye itlelf, or ia
its mufcles, nothing is preternatural or defective.
Thus children by imitating thole that fquint, and
infants by having many agreeable objects prefented tq
them at once, which invite them to turn one eye to
one and the other eye to another, do frequently con¬
tract a habit of moving their eyes differently, which
■afterwards they cannot fo ealily correCt. Infants
likewife get a cuftom of fquinting by being placed
obliquely towards a candle, window, or any other-
agreeable objeCt capable of attracting their fight: for
though, to iee the objeCt, they may at firlt turn both
eyes towards it ) yet, becaufe fuch an oblique fituation
is painful and laborious, efpecially to the molt diftant
©ye, they foon relax one of the eyes, and content
themfelves with examining it with the eye that is
m.ext it •, whence arifes a diverfity of fituation ^nd a
habit of moving the eyes differently.
In this cafe, which may admit of a cure if not too
much confirmed, it is evident, that obje&s will be feen
. in the fame place by both eyes, and therefore muff
appear fingle as to other men ; but becaufe, in the eye
that fquinis, the image of the objeCI to which the
other eye is direCIed falls not on the mold fenfible and
delicate part of the retina, which is naturally in the
axis of the eye, it is cafy to fee that it muff be but.
faintly perceived by this eye. Hence it is, that white
they are attentive in viewing any objeCt, if the hand
be brought before the other eye, this objeCt will be
but obfeurely feen, till the eye change its fituation
and have its axis directed to it ; which change of fitu¬
ation is indeed very eafy for them, becaufe it depends
on the mufcles of the eyes, whofe functions are entire j
but, by reafon of the habit they have contracted’of
moving their eyes differently, the other eye is at the
fame time frequently turned afide, fo that only one at a
time is direCted to this objeCt.
II. The J} mb if thus may proceed from a fault in the
firlt conformation, by which the moft delicate and
leniible part of the retina is removed from its natural
fituation, which is direCtly qppofite to the pupil, and is
placed a little to a fide of the axis of the eye ; which
obliges fuch people to turn away the eye from the objeCt
they would view, that its piClure may fall on this moft
fenfible part of the organ.
When this is the cafe, the difeafe ip altogether in¬
curable, and the phenomena that arife therefrom differ
in nothing from the phenomena of the former cafe,
excepting only that here, r. The objeCt to which the
eye is not direCted will be belt feen *, which is the re-
verfe of what happens when this difeafe arifes barely
from habit and cuftom. 2^ No objeCt will appear
altogether clear and dirtinCt: for all objeCts to which
the eye is direCted, by having their image painted in
the retina at the axis of the eye, where it is not very
fenfible, will be but obfeurely feen ; and objeCts that
are placed fo far to a fide of the optic axis as is ne-
ceffarv for making their image fall on the moft fenfible
and delicate part of the retina, mult appear a little
MEDICI N E. PiaaiM
confufed, becaufe the feveral pencils of rays that come StrabifnnJ
tnmfrom fall, tuo obliquelv on the ervllalline tn ho -v—
tnmfrom fall, tuo obliquely on the cryllalline to be
accurately collected in fo many diftinct points of the
retina-j., though it muff be acknowledged, that this
confuiion will, for the moft part, be fo fmall as toefcape
unobferved.,
HI.. 1 his difeafe may proceed from an oblique po-
fition of the cryftaliine, where the rays that come di,
reClly to the eye from an objeCt, and that ought to
converge to the point of the retina, which is in the
axis of the eye, are, by reafon of the obliquity of the
cryftaliine, made to converge to another point on that
ffoe of the vifual axis where the cryftalline is moft
elevated; and therefore the objeCt is but obfeurely
feen, becaufe its image falls not on the retina at the
axis of the eye, where it is moft fenfible : But the
rays that fall obliquely on the eye, will after refrac¬
tion, converge to this moft fonfible part of the re¬
tina 5 and, by converging there, mull imprefs the
mind with a clear idea of the objeCt from u hence
they came. It is for this reafon that the eye ne¬
ver moves uniformly with the other, but turns away
from the objeCt it would vierv, being attentive to
the objeCt to which it is not direCted. When this is
the. cafe, it is in vain to expect any good from me¬
dicine.
The fymptoms which naturally arife from it are,
i. The objeCt to which the eye is direCted will be
but faintly feen, becaufe its image falls on the retina
where it is not very fenfible. 2. 'I he objeCt to which
the eye is not direCted, by having its image painted
on the retina at the axis of the eye, will be clearly-
perceived.. But, 3. This fame objeCt mult appear
feme what indifiinCt, becaufe the pencils of rays that
flow from it are not accurately collected in fo many
diftinCt points in the retina, by reafon of their oblique
incidence on tile cryftaliine. 4, It muff be feen, not
in its proper place, but thence ttanfiated to fome
other place fituated in the axis of vifion. And, 5. Be¬
ing thus tranfiated from its true place, where it is feen
by the other eye that does not fquint, it muft neceffa-
rily appear double ; and the diftance between the
places of its appearance ,vill be ftill greater, if the
cryftalline of the other eye incline to the contrary
fide.
IV. This difeafe may arife from an oblique pofi-
tion of the cornea p w hich, in this cafe, is gene¬
rally more arched and prominent than what it is na¬
turally.
When the eye has this conformation, no objeCI to
which it is direCted can be clearly feen, becaufe its
image falls not on the retina at the axis of the eye p
and therefore the eye turns afide from the objeCI it-
would view, that its image may fall on the moft fen¬
fible part of the retina.
When the ftrabifmus proceeds from this caufc, the
prognoftic and the phaenomena that attend it will be-
much the fame as in the cafe immediately preceding j
from which neverthelefs it may be diftinguilhed by.
the obliquity of the cornea, which is manifeft to the
fenfes, and if the cornea be alfo more arched and'
prominent than what it is naturally, which is common-'
ly the cafe, the eye will alfo be fhort-fighted.
V. This want of uniformity in the motions of our
eyes, may arife from a, dcfoCI, or any great weaknels
or