565 
OPT 
as at the firft. This has been thought verified by Mr. 
Chefelden ; who informs us, that a gentleman, who, from 
a blow on bis head, had one eye diftorted, found every ob¬ 
ject to appear double; but, by degrees, the molt familiar 
ones came to appear fingle again ; and, in time, all objects 
did fo, without any amendment of the diftortion. 
On the other hand, Dr. Reid is of opinion, that the 
correfpondence of the centres of the two eyes, on which 
fingle vifion depends, does not arife from cuftom, but 
from fome natural conftitution of the eye and of the 
mind. He makes feveral juft objeftions to the cafe of Mr. 
Forfter, recited by Dr. Smith and others ; and thinks, that 
the cafe of the young man couched by Chefelden, who 
faw fingly with both eyes immediately upon receiving his 
fight, is nearly decifive in proof of his fuppofition. He 
alfo found that three young gentlemen, whom he endea¬ 
voured to cure of fquinting, faw objects fingly as foon 
as ever they were brought to direft the centres of both 
their eyes to the fame objeft, though they had never been 
ufed to do fo from their infancy: and he obferves, that 
there are cafes, in which, notwithftanding thefulleft con- 
viftion of an objeft being fingle, no practice of looking at 
it will ever make it appear fo, as when it is feen through a 
multiplying-glafs. 
To all thefie folutions of the difficulty refpefting fingle 
vifion by both eyes, objections have been lately made 
which feern infrirmountable. By judicious experiments. 
Dr. Wells has (liown, that it is neither H 7 ' cuftom alone, 
nor by the original property of the eyes alone,'that objefts 
appear fingle; and, having demolifhed the theories of others, 
he thus endeavours to account for the phenomenon. 
“ The vifible place of an objeft bemg compofed of its 
vifible diftance and vifible direction, to fhow how it may 
appear the fame to both eyes, it will be necefiary (fays he) 
to explain in what manner the diftance and direction 
which are perceived by one eye may coincide with thofe 
which are perceived by the other.” 
With refpeft to vifible diftance, our firft ideas are doubt- 
lefs acquired by the ftretching-out and drawing back of 
our arms ; and thofe ideas are foon fo connected with cer¬ 
tain fenfations which we have in aCtual vifion, that the 
latter inftantly fuggefts the former. Thus it is a faCt 
known by experience, that, when we look at a near objeCt 
with both eyes, according as it approaches or recedes 
from us, w’e alter the difipofition of our eyes, by leffening 
or widening the interval between the pupils. This dif- 
pofition, or turn of the eyes, is attended with a fenfation 
of which every man is confcious at the time of vifion; 
and this fenfation feerns to us to be that which, in this 
cafe, fuggefts the idea of greater or lefs diftance to the 
mind. Not that there is any natural or necefjary con¬ 
nexion between the fenfation of which we are confcious 
and greater or lefs diftance ; for the fenfation is wholly 
internal, and the diftance is external. But, becaufe the 
mind has, by conftant experience, found the different 
fenfations occafioned by different difpofitions of the eyes 
to correfpond to different degrees of diftance in the objeft, 
there has grown an habitual or cuftomary connexion be¬ 
tween thofe fenfations and the notions of greater or lefs 
diftance. So that the mind no fooner perceives the fen¬ 
fation arifing from the different turn it gives the eyes, in 
order to bring the pupils nearer or farther afunder, than 
it is inftantly impreffed with a certain notion of the dif¬ 
tance which was wont to be connected with'that fenfation. 
Again: An objeCt placed at a certain diftance from the 
eye, to which the breadth of the pupil bears a fenfible 
proportion, being made to approach nearer, is feen more 
confufedly ; and the nearer it is brought, the ccnfufion 
is always the greater. The reafon of all this is known 
to the optician : but, it being conftantly experienced by 
thofe who never dipt into optics, there arifes in the mind 
of every man an habitual connexion between the feveral 
degrees of confufion and diftance ; the greater confufion 
ftill implying the lefs diftance, and the lefs confufion the 
greater diftance. In thefe ways, however, we perceive 
Vol. XVII. No. 1198. 
ICS. 
only fmall diftances. Of diftances more remote, our judg* 
ment is formed from other data ; and happily, thefe data 
are not fartofeek. It is an axiom in optics, that a greater 
number of rays fall upon the eye when refleCted f rom a 
body near at hand, than can fall from the fame body at a 
diftance; and, as thofe rays operate by impulfe, it is felf- 
evident that the impreffion muft be ftronger, and of courfe 
the fenfation more vivid, when the body is near than 
when it is diftant. New, having acquired the notion of 
the true diftance of objefts by motion and the fenfe of 
touch, and finding by uniform experience, that, as they 
are near or far off, the fenfation or colour which they 
excite in the mind through the organ of vifion is more or 
lefs vivid, thofe degrees of fenfation come to be fo clofely 
affociated with the refpeftive diftances of the objeft, that 
the one inftantly fuggefts the other. 
As to vifible direftion; when a fmall objeft is fo placed 
with refpeft to either eye, as to be feen more diftinftiy 
than in any other fituation, our author fays, that it is then 
in the optic axis, or the axis of that eye. When the two 
optic axes are directed to a fmall objeft not very diftant, 
they may be conceived to form two tides of a triangle, of 
which the bafe is the interval between the points of the 
corners where the axes enter the eyes. This bafe he called 
the vifualbafe; and a line drawn from the middle of it 
to the point of interfeftion of the optic axis he calls the 
common axis. He then proceeds to fhow, that objects 
really Jituated in the optic axis do not appear to be in that 
line, but in the common axis. Every perfon (heobferves) 
knows, that, if an object be viewed through two fmall 
holes, one applied to each eye, the two holes appear but 
as one. The theories hitherto invented afford two expla¬ 
nations of this faft. According to Aguilonius, Dechales, 
Dr. Porterfield, and Dr. Smith, the two holes, or rather 
their borders, will be feen in the fame place as the objeft 
viewed through them, and will, confequently, appear 
united, for the fame reafon that the object itfelf is feen 
fingle. But whoever makes the experiment will diftinftiy 
perceive, that the united hole is much nearer to him than 
the objeft ; not to mention, that any fallacy on this head 
might be corrected by the information from the fenfe of 
touch, that the card or other fubftance in which the holes 
have been made is within an inch or lefs of our face. The 
other explanation is that furniftied by the theory of Dr. 
Reid. According tp it, the centres of the retinas, which 
in this experiment receive the pictures of the holes, will, 
by an original property, reprefent but one. This theory, 
however, though it makes the two holes to appear one, 
does not determine where this one is to be feen. It can¬ 
not be feen in only one of the perpendiculars to the 
images upon the retinas, for no reafon can be given why 
this law of vifible direftion, which Dr. Reid thinks eftab- 
lifiied beyond difpute, if it operates at all, ftiould notope- 
rate upon both eyes at the lame time; and, if it be feen 
by both eyes in fuch lines, it muft appear where thofe lines 
crofs each other, that is, in the fame place with the object 
viewed through the holes, which, as already mentioned, 
is contrary to experience. Nor is it feen in any direftion, 
the confequence of a law affefling both eyes confidered 
as one organ, but fufpended when each eye is ufed fepa- 
rately. For, when the two holes appear one, if we pay 
attention to its fituation, and then clofe one eye, the truly 
fingle hole will be feen by the eye remaining open in ex- 
aftly the fame direftion as the apparently fingle hole was 
by both eyes. 
“ Hitherto I have fuppofed the holes almoft touching 
the face. But they have the fame unity of appearance, 
in whatever parts of the optic axis they are placed ; whe¬ 
ther both be at the fame diftance from the eyes, or one 
be clofe to the eye in the axis of which it is, and t he other 
almoft contiguous to the objeft feen through them. If 
a line, therefore, be drawn from the objeft to one of the 
eyes, it will reprefent all the real or tangible poiitions of 
the hole which allow the objeft to be feen by that eye, 
and the whole of it will coincide with the optic axis. Let 
7 E a fimilar 
