OPT 
M. Marriotte obferves, that this improvement on his 
experiment, by M. Picard, is ingenious, but difficult to 
execute, iince the eyes mult be confiderably drained in 
looking at any objedt fo near as four inches; and propofes 
another, not lefs furprifing, and more ealy. Place, fays 
lie, on a dark ground, two round pieces of white paper, 
at the fame height, and three feet from one another; then 
ftand oppofite to them, at the diftance of twelve or thir¬ 
teen feet, and hold your thumb before your eyes, at the 
diftance of about eight inches, fo that it may conceal 
from the right eye the paper that is to the left hand, and 
from the left eye the paper to the right hand. Then, if 
you look at your thumb fteadily with both eyes, you will 
lofe fight of both the papers ; the eyes being fo difpofed, 
that each of them receives the image of one of the papers 
upon the bafe of the optic nerve, while the other is inter¬ 
cepted by the thumb. 
M. Le Cat purfued this curious experiment a little far¬ 
ther than M. Marriotte. In the place of the fecond paper, 
lie fixed a large white board, and obferved, that at a pro¬ 
per diftance he loft fight of a circular fpace in the centre of 
it. He all’o obferved the fize of the paper which is thus 
concealed from the fight correfponditig to feveral dif- 
tances, which enabled him to afeertain various circum- 
ftances relating to this part of the ttrudture of the eye 
more exadtly than had been done before. 
The following is the manner in which this curious ex¬ 
periment is now generally made. Let three pieces of 
paper be fattened upon the fide of a room, about two feet 
afun.der; and let a perfon place himfelf oppofite to the 
middle paper, and, beginning near to it, retire gradually 
backwards, all the while keeping one of his eyes lliut, and 
the other turned obliquely towards that outfide paper 
which is towards the covered eye ; and he will find a fitua- 
tion (which is generally at about five times the diftance 
at which the papers are placed from one another), when 
the middle paper will entirely difuppear, while the two 
outermoft continue plainly vilible ; becaufe the rays which 
come from the middle paper will fall upon the.retina 
where the optic nerve is inferted. 
The dimenfions and precife form of the fpot in the eye 
in which there is no vifion, were more accurately calcu¬ 
lated by Daniel Bernouilii, in the following manner: 
He placed a piece of money, O, (fig. n.) upon the floor ; 
and then Ihuttingone of his eyes, and making a pendulum 
to f'wing fo that the extremity of it might be nearly in 
the line AO, he obferved at what place C it began to be 
invifible, and where it again emerged into view' at A. 
Shifting the pendulum higher and lower, he found other 
points, as H, N, P, G, B, at which it began to be invifi¬ 
ble again ; and others, as M, L, E, A, at which it began 
to be vilible again; and, drawing a curve through them, 
lie found that it was elliptical ; and, with refpedt to 
his own eye, the dimenfions of it were as follow : OC 
was 23, AC 10, BD 3, DH 13, and EG 14; fo that, the 
centre being at F, the greater axis was to the lefs as 
8 to 7. From thefe data, the plane on wffiich the figure 
was drawn being obliquely fituated with refpedt to the 
eye, he found, that the place in_ the eye that corref- 
ponded to it was a circle, the diameter of which was 
a feventh part of the diameter of the eye; the centre 
of it being 27 parts of the diameter from the point op¬ 
pofite to the pupil, a little above the middle. In order, 
therefore, that this fpace, in which there is no vifion, 
may be as fmall as polfible, it is neceflary that the nerve 
ihould enter the eye perpendicularly, and that both this 
end, and alfo its entering the eye at a diftance from its 
axis, are gained by the particular manner in which the 
two optic nerves unite and become feparate again, by 
crofting one another. 
Dr. Porterfield obferves, that the reafon w'hy there is no 
vifion at the entrance of the optic nerve into the eye, may 
be- its want of that foftnefs and delicacy which it has 
when it is expanded upon the choroides; and that, in 
£hofe animals in which that nerve is inferted in the axis 
Vol. XVII. No. 2199. 
ICS. 585 
of the eye, it is obferved to be equUll)' delicate, and there¬ 
fore probably equally fenfible, in that place as in any other 
part of the retina. In general, the nerves, when em¬ 
braced by their coats, have but little fenfibility in compa¬ 
nion of what they are endued with when they are di vetted 
of them, and unfolded in a loft and pulpy lubftance. 
Of the apparent Place, Diftance, Magnitude, and Motion, 
of Objedt s. 
It had in general been taken for granted, that the place 
to which the eye refers any vilible objedt feen by reflec¬ 
tion or refraction, is that in which the vifual rays meet a 
perpendicular from the objedt upon the reflecting or re¬ 
fradting plane. But this method of judging of the place 
of objedts was called in queltion by Dr. Barrow, who con¬ 
tended that the arguments brought in favour of the opi¬ 
nion were not concluiive. Thefe arguments are, that the 
images of objedts appeared ftraight in a plane mirror, 
but curved in a convex or concave one ; that a ftraight 
thread, when partly irnmerfed perpendicularly in water, 
does not appear crooked, as when it is obliquely plunged 
into the fluid; but that which is within the water feems 
to be a continuation of that which is without. With re- 
fpedt to the refiedted image, however, of a perpendicular 
right line from a convex or concave mirror, he fays, that 
it is not eafy for the eye to diftinguiffi the curve that it 
really makes ; and that, if the appearance of a perpendi¬ 
cular thread, part of which is irnmerfed in water, be 
ciofely attended to, it will not favour the common hypo- 
thelis. If the thread is of any Alining metal, as filver, and 
viewed obliquely, the image of the part irnmerfed will 
appear to detach itfelf fenlibly from that part which is 
without the water, fo that it cannot be true, that every 
objedt appears to be in the fame place where the refradted 
ray meets the perpendicular; and the fame obfervation, 
he thinks, may be extended to the cafe of refledtion. 
According to Dr. Barrow, we refer every point of an ob¬ 
jedt to the place from which the pencils of light, that give 
us the image of it, iflue, or from which they would have 
iffued if no reflediing or refradting fubftance intervened. 
Purfuing this principle, he proceeds to inveftigate the 
place in which the rays iftuing from each of the points of 
an objedt, and which reach the eye after one refledtion or 
refradtion, meet; and he found, that, if the refradting fur- 
face was plane, and the refradtion was made from a denfer 
medium into a rarer, thofe rays would always meet in a 
place between the eye and a perpendicular to the point of 
incidence. If a convex mirror be ufed, the cafe will be 
the lame ; but, if the mirror be plane, the rays will meet 
in the perpendicular, and beyond it if it be concave. He 
alfo determined, according to thefe principles, what form 
the image of a right line will take, when it is prefented in 
different manners to a fpherical mirror, or when it is feen 
through a refradting medium. 
Though Dr. Barrow reckoned the maxim which he en¬ 
deavoured to eltablilh, concerning the fuppofed place of 
vilible objects, highly probable, he has the candour to 
mention an objedtion to it, of which he was notable to 
give a latisfadlory folution. It is this. Xet an objedt be 
placed beyond the focus of a convex lens; and, if the eye 
be clofe to the lens, it will appear confuted, but very near 
to its true place. If the eye be a little withdrawn, the 
confufion will increafe, and the objedt will feem to come 
nearer; and, when the eye is near the focus, the confu¬ 
fion will be exceedingly great, and the objedt will feem to 
be clofe to the eye. But, in this experiment, the eye re¬ 
ceives no rays but thofe that are converging; and the 
point from which they iflue is fo far from being nearer 
than the objedt, that it is beyond it; notwithltandin^ 
which, the objedt is conceived to be much nearer than 
it is, though no very diltindt idea can be formed of 
its precife diftance. It may be obferved, that, in reality, 
the rays falling upon the eye in this cafe in a manner 
quite different from that in which they fall upon it in 
other circumftances, we can form no judgment about the 
7 K- place 
