584 
O P 
ICS. 
pi flu re upon the retina varies as the angle which the 
objeft fubtends at the centre of the eye nearly; that is, 
nearly in the inverfe ratio of the diftance of the object 
from the eye ; confequently, the area of the pifture upon 
the retina, or fpace over which the rays are diffuJed, 
varies invedely as the fquare of that difiance nearly. 
Hence it follows, that the denfity of rays in the image 
varies invedely as the fquare of the diftance of the ob¬ 
ject from the eye on one account, and direftly as the 
fquare of that diftance on the other ; therefore, upon the 
whole, the denfity is invariable. 
What has been proved of the image of one fmall por¬ 
tion of the objedt, may be proved of every other; confe- 
.quentjy, the denfity, in every part of the image upon the 
retina, is invariable. 
It may here be obferved, that a conftderable quantity 
of light is loft, or abforbed, in its paffage through the 
air; and that the quantity thus loft, cateris paribus, in- 
creafes as the diftance between the objeft and the eye in- 
creafes, though notin that ratio: for, if the (paces through 
which the light pafi'es increase in arithmetical progreftion, 
the quantity of light will decreafe in geometrical pro- 
greflion. On this account, therefore, the brightnefs of 
an object decreafes as its diftance from the eye increafes. 
As the diftance of the objeft increafes, however, the aper¬ 
ture of the pupil is enlarged ; and, therefore, more rays 
are, by this means, received into the eye; and thus the 
former eft eft is in fome degree counteracted. 
Did the denfity of rays in the picture upon the retina 
decreafe confiderably, as the diftance of the objeft in¬ 
creafes, bodies in the neighbourhood of the fpeftator 
would, by their fuperior brightnefs, overpower the im¬ 
preffions made by thofe which are more remote ; and the 
latter would be difcerned with great difficulty, or not at 
all. We are indeed able to diftinguifh objects in exceed- 
ingly-dift'erent degrees of light at different times : thus 
we are able to read a fmall print by moonlight, though its 
intenfity does not exceed one 90,000th part of the inten- 
fity of common day-light; (Dr. Smith. M. Bouguer 
fays, one 300,000th part.) But this quantity of light is 
not fufficient to render fuch objefts dil'cernible as., are fur- 
rounded by others much more luminous; for the ftrong 
light proceeding from the latter bodies, by the powerful 
impreflion it makes upon the retina, overcomes the c-ffefl 
produced upon the more delicate pencils which flow from 
the former, as weaker founds are not diftinguiftiable in a 
hurricane. This feems to be the reafon that the flame of 
a candle is fcarcely dil’cernible in broad day-light; and 
that liars become vifible at different times after fun-fet, 
according to their different degrees of brightnefs. 
The impreffions made by rays of light upon the retina 
continue fome time after the impulfes ceafe, as appears 
by the experiment of a burning coal whirled round in a 
circle. Sir Ifaac Newton accounts for this phenomenon, 
by fuppofing that the impreffions of light are conveyed to 
the brain by vibrations excited in the retina, and propagat¬ 
ed, through the optic nerve, to the fenforium; and that 
the vibrations, once produced, continue fome time, perhaps 
about a fecond, after the exciting caufe has cealed to aft. 
In explaining the nature and circumftances of vifion, 
we have only to attend to the flrufture of one eye ; for, 
in whatever manner rays are refrafled and images formed 
by the humours of one eye, in the fame manner wiil the 
fame effefts be produced by the humours of the other. 
The only queftion that can arife is, how it happens 
that, in vifion with both eyes, objefts appear Angle. It 
is not eafy to decide whether this eft'eft is produced by 
the funilarity of correfponding parts of the optic nerves, 
and their union in the brain, or by habit. To the rea¬ 
soning upon this Subject at p. 564, we may add, that 
children fometimes learn to fquint; and, by proper at- 
tention, this habit may again, in a great meafure, be 
correfted. Under both circumftances, objefts appear An¬ 
gle; and it is manifeft that the images cannot, in both 
•cafes, fill upon correfponding points of the retinas. 
But, whatever be the caufe of Angle vifion, when an 
objeft is viewed attentively, the axes of both eyes are, in 
general, direfted to it. Thus, if P (fig. 7.) be the objeft, 
the eyes are moved till the optic axes, AP, a P, meet in 
P; and the images A, a, are formed on correfponding 
points of the retinas. In this polifion of the images, whe¬ 
ther from the correfpondence of the nerves, or from expe¬ 
rience, the idea of a Angle objeft is fuggefted to the mind ; 
fcarcely differing from the idea excited by one of the 
images alone, excepting that the objeft appears fomewhat 
brighter when feen with both eyes than when feen with 
one. Alfo, vvhilft the eyes remain in the fame pofition, 
the images, B, b, of Q, an obieft near to P, and at the 
fame diftance from the eyes, will be formed on the retinas; 
the eyes having affumed a proper conformation for dif- 
tindl vifion at that diftance ; and B, 'which are both on 
the right, or both on the left, of the refpedlive axes, are 
correfponding points, and fuggeft the idea of a Angle ob- 
jedf, as in the former cafe. 
But, if an objedt Q (fig. 8.) lie between the optic axes, 
or thofe axes produced, its images will be formed at B, b , 
on points upon the retinas which do not correfpond ; anti 
thus they will excite the fenfations ufually produced by 
different objedts, D, E, at that diftance to which the eyes 
are adapted for vifion. However, when the attention is 
more particularly called to the object Q, the optic axes 
are directed to it, and the points, D, E, coincide. See 
fig. 9. 
As the fenfe of feeing is allowed to beoccafioned by the 
impuife of the rays from the vifible objedt upon the reti¬ 
na, and thus forming the image of the objedt upon it, 
and that the retina is only the expaniion of the optic nerve 
ail over the choroides, it fhould Seem furprifmg, that the 
part of the image which falls on the optic nerve fhould 
render the like part of the objedt invisible ; efpecially as 
that nerve is allowed to be the inftrument by which the 
impuife and image are conveyed to the common SenSory 
in the brain. But, that part of the image which falls 
upon the middle of the optic nerve is loll, and conf'e- 
quently the correfponding part of the objedt is rendered 
inviftble, is plain by experiment. For, if a perfon fixes 
three patches, A, B, C, (fig. 10.) upon a white wall, at 
the height of the eye, and at the diftance of about a foot 
from each other, and places himfelf before them, (hutting 
the right eye, and diredting the left towards the patch C, 
he will fee the patches A and C, but the middle patch B 
will difappear. Or, if he (huts his left eye, and diredts 
the right towards A, he will fee both A and C, but B will 
difappear; and, if he diredts his eye towards B, he wiil 
fee both B and A, but not C. For whatever patch is di- 
redtly oppoftte to the optic nerve N vanifties. This re¬ 
quires a little pradiice; after which he will find it eafy to 
diredt his eye So as to lofe the fight of whatever patch he 
pleafes. 
This experiment, firft tried by M. Marriotte, occafioned 
a new hypothefis concerning the feat of vifion, which he 
iuppofed not to be in the retina, but in the choroides. 
An improvement on the experiment was afterwards made 
by M. Picard, who contrived that an objedt (hould dif¬ 
appear when both the eyes were kept open. He faftened 
upon a wall a round white paper, an inch or two in dia¬ 
meter ; and, by the fide of it he fixed two marks, .one on 
the right hand, and the other on the left, each at' about 
two feet diftance from the paper, and fomewhat higher. 
He then placed himfelf diredtly before the paper, at the 
diftance of nine or ten feet, and, putting the end of his 
finger over-againft both his eyes, fo that the left-hand 
mark might be hid from the right eye, and the right- 
hand mark from the left eye. Remaining firm in this 
pofture, and looking fteaclily, with both eyes, on the. end 
of ]iis finger, the paper which was not at all covered by 
it would totally difappear. This, he fays, is the more 
furprifmg, becaufe, without this particular encounter of the 
optic nerves, where no vifion is made, the paper will appear 
double, as is the cafe when the finger is not rightly placed. 
M. Marriotte 
