5 GO OPT 
a fimilar line be drawn to the othereye, and tbe two mud 
appear but as one line ; for, if they do not, the two l^oles 
in the optic a\es will not, at every diftance, appear one, 
whereas experiments prove that they do. This united 
line will therefore reprefent the vifible direction of every 
object fituated in either of the optic axes. But the end 
of it which is toward the face, is feen by the right eye 
to the left, and by the left eye as much to the right. It 
mult be feen, then, in the middle between the two ; and, 
confequently, in the common axis. And, asits other ex¬ 
tremity coincides with the point where the optic axes in- 
terfeft each other, the whole of it mull lie in the common 
axis. Hence the truth of the proportion is evident, that 
objefts fituated in the optic axis, do not appear to be in 
that line, but in the common axis.” 
He then proves by experiments, that objefts fituated in 
the common axis do not appear to be in that line, but in 
the axis of the eye by which they are not feen : that is, 
an objeft fituated in the common axis appears to the right 
eye in the axis of the left, and vice verfa. His next pro¬ 
portion, proved likewife by experiments, is, that “objefts, 
fituated in any line drawn through the mutual interfeflion 
of the optic axes to the vifual bafe, do not appear to be 
in that line, but in another, drawn through the fame in¬ 
terfeflion, to a point in the vifual bafe diftant half this 
bafe from the fimilar extremity of the former line, towards 
the left if the objefts be feen by the right eye, but towards 
the right if feen by the left eye. 
From thefe proportions he thus accounts for fingle vi- 
fion by both eyes : “ If the queftion be concerning an 
objeft at the concourfe of the optic axes, it is feen fingle, 
becaufe its two fimilar appearances, in regard to lize, 
Ihape, and colour, are feen by both eyes in one and the 
fame direftion, or, if you will, in two direftions which 
coincide with each other through the whole of their ex¬ 
tent. It therefore matters not whether the diftance be 
truly or falfely ellimated ; whether the objefl be thought 
to touch our eyes, or to be infinitely remote. And hence 
tve have a reafon, which no other theory of vifible direc¬ 
tion affords, why objefls appeared fingle to the young 
gentleman mentioned by Mr. Chelelden immediately 
after his beingcouched, and before he could have learned 
to judge of diftance by light. 
“When two fimilar objefls are placed in the optic axes, 
one in each, at equal diltances from the eyes, they will 
appear in the fame place, and therefore one, for the fame 
reafon that a truly fingle objefl, in the concourfe of the 
optic axes, is feen fingle. 
“ To finifh this part of my fubjefl, it feems only necef- 
fary to determine, whether the dependence of vifible di¬ 
reftion upon the aftions of the mufcles of the eyes be 
eftablilhed by nature, or by cuftoin. But fafts are here 
wanting. As far as they go, however, they ferve to prove 
that it arifes from an original principle of our conftitution. 
For Mr. Chefelden’s patient law objefts fingle, and con¬ 
fequently in the fame direftions, with both eyes, immedi¬ 
ately after he was couched ; and perfons affefted with 
fquinting from their earlieft infancy, fee objefls in the fame 
direftions with the eye they have never been accuftomed 
to employ, as they do with the other they have conftantly 
■ufed.” 
We are indebted to Dr, Jurin for the following curious 
experiments, to determine whether an objeft feen by both 
eyes appears brighter than when feen with one only. 
He laid a Hip of clean white paper direftly before him 
on a table ; and, applying the fide of a book clofe to his 
right temple, fo that the book was advanced confiderably 
farther forward than his face, he held it in'fuch a manner, 
as to hide from his right eye that half of the paper which 
lay to his left hand, while the left half of the paper was 
feen by both eyes, without any impediment. Then, look¬ 
ing at the paper with both eyes, he obferved it to be di¬ 
vided, from the to*p to the bottom, by a dark line, and 
the part which was feen with one eye only was manifeftly 
darker than that which was leen with both eyes; and, 
I c s. 
applying the book to his left temple, he found, by the 
refult of the experiment, that both his eyes were of equal 
goodnefs, 
He then endeavoured to determine the excefs of this 
brightnefs ; and, comparing it with the appearance of an 
objefl illuminated partly by one candle and partly by 
two, he was furprifed to find that an objeft feen with 
two eyes is by no means twice as luminous as when it is 
feen with one; and, after a number of trials, he found, 
that, when one paper was illuminated by a candle placed 
at the diftance of three feet, and another paper by the 
fame candle at the fame diftance, and by another candle 
at the diftance of eleven feet, the former feen by both 
eyes, and the latter with one eye only, appeared to be 
of equal whitenefs ; fo that an objefl feen with both eyes 
appears brighter than when it is feen with one only by 
about a 13th part. 
He then proceeded to inquire, whether an objefl feen 
with both eyes appears larger than when feen with one ; 
but he concluded that it did not, except on account of 
forne particular circumftances, as in the cafe of the bino¬ 
cular telefcope and the concave fpeculum. 
M. du Tour maintains, that the mind attends to no 
more than the image made in one eye at a time ; and pro¬ 
duces feveral curious experiments in favour of this hypo- 
thefis, which had alfo been maintained by Kepler, and 
almoft all the firil opticians. But, as M. Buffon obferves, 
it is a fufticient anfwer to this hypothefis, however inge- 
nioufly it i\*.j bt fupported, that we fee more diftinftly 
with two eyes than wfith one ; and that, when a round 
objeft is near us, we fee more of the furface in one cafe 
than in the other. 
With refpefl to fingle vifion with two eyes, Dr. Hart¬ 
ley obferves, that it deferves particular attention, that 
the optic nerves of men, and fuch other animals as look the 
fame way with both eyes, unite in the cella turcica in a gan¬ 
glion, or little brain, as one may call it, peculiar to them- 
felves ; and that the aftociations between fynchronous 
imprefiions on the two retinas mull be made fooner and 
cemented ftronger on this account: alfo, that they ought 
to have a much greater power over one another’s images, 
than in any other part of the body. And thus an impref- 
fion made on the right eye alone by a fingle objefl, may 
propagate itfelf into the left, and there raile up an image 
almoft equal in vividnefs to itfelf; and, confequently, 
when we fee with one eye only, we may, however, have 
pictures in both eyes. 
A curious deception in vifion, arifing from the ufe of 
both eyes, was oblerved and accounted for by Dr. Smith. 
It is a common obfervation, he fays, that objefls feen 
with both eyes appear more vivid and ftronger than they 
do to a fingle eye ; efpecially when both of them are 
equally good. A perfon, not ftiort-fighted, may loon be 
convinced of this faff, by looking attentively at objefls 
that are pretty remote, firft with one eye, and then with 
both. This obfervation gave occafion to the conftruftion 
of the binocular telefcope, in the ufe of which the phe¬ 
nomenon is ftill more ftriking. 
A curious circumllance relating to the effeft of one eye 
upon the other, was noticed by M. JEpinus, who ob¬ 
ferved, that, when he was looking through a hole made 
in a plate of metal, about the 10th part of a line in dia¬ 
meter, with his left eye, both the hole itfelf appeared larger, 
and alfo the field of view feen through it was more ex¬ 
tended, whenever he Ihut his right eye ; and both thefe 
effefts were more remarkable when that eye was covered 
with his hand. He found confiderable difficulty in mea- 
furing this augmentation of the apparent diameter of the 
hole, and of the field of view; but at length he found, 
that, when the hole was half an inch, and the tablet 
which he viewed through it was three feet from his eye, 
if the diameter of the field when both his eyes were open 
was i, it became if when the other eye was Ihut, and 
nearly z when his hand was laid upon it. Upon examin¬ 
ing this phenomenon, it prefently appeared to depend 
upon 
