58G - OPT 
place from which they iffue. Thisfubjedl was afterwards 
taken up by Berkeley, Smith, Montucla, and others. 
M. de la Hire made feveral obfervations concerning the 
diftance of vilible objedfs, and various other phenomena of 
vilion, which are worthy of notice.' He alfo took parti¬ 
cular pains to alcertain the manner in which the eye con¬ 
forms itfelf to the view of objects placed at different dif¬ 
tances. He enumerates five circumftances which aflift us in 
judging of the diftance of objects; namely, their apparent 
magnitude, the ftrength of.the colouring, the diredtion 
of the tw'o eyes, the parallax of the objedls, and the dif- 
tindtnefs of their fmall parts. Painters, he fays, can only 
take advantage of the tw'o firft-mentioned circumftances, 
and therefore pi cl ures can never perfedtly deceive the eye; 
but, in the decorations of theatres, they, in fome meafure, 
make ufe of them all. The fixe of objedfs, and the ftrength 
of their colouring, are diminifhed in proportion to the 
diftance at which they are intended to appear. Parts of 
the fame obje<ft which are to appear at different diftances, 
as columns in an order of archiledtlire, are drawn upon 
different planes a little removed from one another, that 
the two eyes may be obliged to change their diredlion, in 
order to diftinguilh the parts of the nearer plane from 
thofe of the more remote. The fmall diftance of the 
planes ferv.es to make a fmall parallax, by changing the 
pofition of the eye ; and, as we do not preferve a diftindt 
idea of the quantity of parallax correfponding to the 
different diftances of objedfs, it is fufficient that we per¬ 
ceive there is a parallax, to be convinced that thefe planes 
are diftant from one another, without determining what 
that diftance is ; and, as to the laft circumftance, viz. the 
diftindtnefs of the fmall parts of objedfs, it is of no ufe in 
difcovering the deception, on account of the falfe light 
that is thrown upon thefe decorations. 
To thefe obfervations concerning deception of fight, 
we ftiall add a fimilar one of M. le Cat, who took notice, 
that the reafon why w.e imagine objedfs to be larger when 
they are feen through a mill, is the dimnefs or obfcurity 
with which they are then feen; this circumftance being 
affociated with the idea of great diftance. This, he fays, is 
confirmed, by our being furprifed to find, upon approach¬ 
ing fuch objedfs, that they are fo much nearer to us, as 
well as fo much fmaller, than we had imagined. 
Among other cafes concerning vifion, which fell under 
the confederation of M. de la Hire, he mentions one which 
it is difficult to folve. It is when a candle, in a dark 
place, and fituated beyond the limits of diftindt vifion, is 
viewed through a very narrow chink in a card ; in wdiich 
cafe, a confiderable number of candles, fometimes fo many 
as fix, will be feen along the chink. This appearance he 
afcribes to fmall irregularities in the furface of the hu¬ 
mours of the eye, the effedt of which is not f’enfible when 
rays are admitted into the eye through the whole extent 
of the pupil, and confequently one principal image effaces 
a number of fmall ones; whereas, in this cafe, each of 
them is formed feparately, and no one of them is fo confi¬ 
derable as to prevent the others from being perceived at 
the fame time. 
There are few perfons, M. de la Hire remarks, who 
have both their eyes exadfly equal, not only with refpedf 
to the limits of diftindt vifion, but alfo with regard to the 
colour with which objedfs appear tinged when they are 
viewed by them, efpeciallyif one of the eyes has been ex- 
poled to the impreftion of aftronglight. To compare 
them together in this refpedf, he directs us to take two 
thin cards, and to make in each of them a round hole of 
a third or a fourth of a line in diameter, and, applying 
one of them to each of the eyes, to look through the holes 
on a white paper, equally illuminated, when a circle of 
the paper wall appear to each of the eyes ; and, placing the 
cards properly, thefe two circles may be made to touch 
one another, and thereby the appearance of the fame ob- 
jedt to each of the eyes may be compared to the greateft: 
advantage. To make this experiment with exadfnels, it 
is neceffary, he fays, that the eyes be kept fhut fome time 
before the cards be applied to them. 
I c s. 
By the following calculation, M. de la Hire gives us an 
idea of the extrerhe fenfibility of the optic nerves. One 
may fee very eafily, at the diftance of 4000 toifes, the fail 
of a windmill fix feet in diameter; and, the eye being fup- 
pofed to be an inch in diameter, the picture of this fail, 
at the bottom of the eye, will be i-8oooth of an inch, 
which is lefs than the 666th part of a line, and is about 
the 66th part of a common hair, or the 8 th part of a fingle 
thread of iiik. So fmall, thefefore, mult one of the fibres 
of the optic nerve be, which,-he fays, is almolt incon¬ 
ceivable, fince each of thefe fibres is a tube that contains 
fpirits. 
That w'e do not judge of diftance merely by the angle 
under which objedfs are feen, is an oblervation as old & as 
Alhazen, who mentions feveral inftances in which, though 
the angles under which objedfs appear be different, the 
magnitudes are univerfally and inftantaneoufiy deemed 
not to be fo. But the followdng experiment is the molt 1 
convincing proof that the apparent diftance of the image 
is not determined by its apparent magnitude. If a double 
convex-glafs be held upright before fome luminous ob- 
jedf, as a candle, there will be feen two images, one eredt 
and the other inverted : the firft is made limply by reflec¬ 
tion from the neareft furface, the fecotui by refledfion from 
the farther furface, the rays undergoing a refradtion from 
the firft furface both before and after the refledfion. If 
this glafs has not too fhort a focal diftance when it is held 
near the objedf, the in verted image will appear larger than 
the other, and alfo nearer; but, if the glafs be carried off 
from the objedf, though the eye remain as near to it as 
before, the inverted image will diminilh fo much falter 
than the other, that at length it will appear very much 
lefs than it, but (till nearer. Here, fays Mr. Robins, two 
images of the fame object are feen under one view-; and 
their apparent diftances, when immediately compared, 
feem to have no neceffary connedfion with the apparent 
magnitude. He alfo ihows how this experiment may be 
made (till more convincing, by Iticking a pieceof paper on 
the middle of the lens, and viewing it through a ihort tube. 
M. Bouguer adopts the general maxim of Dr. Barrow, 
in fuppofing that we refer objedfs to the place from which 
the pencils of rays feemingly converge at their entrance 
into the pupil. But, when rays iffue from below the fur- 
face of a veifel of w'ater, or any other refracting medium, 
he finds that there are always two different places of this 
leeming convergence : one of them, of the rays that iffue 
from it in the fame vertical circle, and therefore fall with 
different degrees of obliquity upon the furface of the re¬ 
fracting medium; and another, of thofe that fall upon 
the furface with the fame degree of obliquity, entering 
the eye laterally with refpedf to one another. Sometimes, 
he fays, one of thefe images is attended to by the mind, 
and fometimes the other ; and different images may be ob- 
ferved by different perfons. An objedf immerfed in w'ater 
affords an example, he fays, of this duplicity of images. 
If BA& (fig. 12.) be part of the furface of water, and 
the objedf be at O, there will be two images of it in two 
different places: one at G, on the caultic by refradfion ; 
and the other at E, in the perpendicular AO, which is as 
much a cauftic as the other line. The former image is 
vifxble by the rays O.DM, O dm, which are one higher than 
the other, in their progrefs to the eye; whereas the image 
at E is made by the rays ODM, Oef, which enter the eye 
laterally. This, fays he, may ferve to explain the diffi¬ 
culty of Tacquet, Barrow, Smith, and many other authors. 
Dr. Porterfield gives a dilfindl view of the natural me¬ 
thods of judging concerning the diftance of objedfs. 
1. The conformation of the eye, he obferves, can be of 
no ufe to us with refpedf to objedfs placed without the li¬ 
mits of diftindt vifion. As the object, however, does 
then appear more or lefs confufed, according as it is more 
or lefs removed from thofe limits, this confufion aflifts the 
mind in judging of the diftance of the objedt; it being 
always eftimated fo much the nearer, or the farther off, as 
the confufion is greater. But this confufion hath its limits 
alfo 5 fan when an objedt is placed at a certain diftance 
from 
