5{?7 
OPTICS. 
upon the enlargement of the pupil of one eye when the 
other is clofed, the phyfical caufe of which he did not 
pretend to affign f but, he obferves, that it is wifely ap¬ 
pointed by Providence, in order that, when one eye fails, 
the field of view in the other may be extended. That 
this effeCt fhould be more fenfible when the eye is covered 
with the hand, is. owing, he obferves, to the eye-lids not 
being impervious to the light. But the augmentation of 
the pupil does not enlarge the field of view, excppt in 
looking through a hole, as in this particular cafe ; and 
therefore perfons who are blind of one eye can derive no 
advantage from this circumdance. 
A great deal has been written by Gaffendi, Le Clerc, 
Muffchenbroek, atid Du Tour, concerning the place to 
which we refer an objeCt viewed by one or both eyes. 
But the moft fatisfaflory account of this matter that we 
have met with, will be found in Dr. Wells’s “ Elfay on 
Single Vifion,” above quoted. 
We {hall conclude this inquiry with the five following 
particulars, which either have been already mentioned, 
or follow from what has been before laid down. 
i. That, as each point of an objeCt, when viewed by 
the naked eye, appears in its proper place, and as that 
place is always to be found in the line in which the axis 
of a pencil of rays, flowing from it, enters the eye, or elfe 
in the line which Dr. Wells calls the common axis; we 
hence acquire a habit of conlidering the point to be fitu- 
ated in that line : and, becaufe the mind is unacquainted 
with what refra&ions the rays fuffer before they enter the 
eye, therefore, in cafes where they are diverted from their 
natural courfe, by palling through any medium, it judges 
the point to be in that line produced.back in which the 
axis of a pencil of rays flowing from it is fituated, the in¬ 
fant they enter the eye, and not in that it was before re- 
^f^That we are able to judge, though imperfectly, of 
the diftance of an objeCt, by the degree of divergency, 
wherein the rays flowing from the fame point of the ob¬ 
ject enter the pupil of the eye, in cafes where that diver¬ 
gency is confiderable ; but, becaufe it will be necefiary 
to fuppofe an objeCt, when feen through a medium 
whereby its apparent diftance is altered, to appear infome 
determinate fituation, in thofe cafes where the divergency 
of the rays at their entrance into the eye is confiderable, 
we will fuppofe the object to appear where thofe lines 
which they defcribe in entering, if produced back, would 
crofs each other : though it mull not be alferted, that 
this is the precife diftance; becaufe the brightnefs, dif- 
tin6lnefs,and apparent magnitude, of the objeCt, on which 
its apparent diftance in Tome meafure depends, will alfo 
fuffer an alteration by the refraCtion of the rays in pafling 
through that medium. 
3. That we eftimate the magnitude of an object by that 
of the optic angle. 
4. That vifion is the brighter, the greater the number 
of rays is which enter the pupil. 
5. And that, in fome cafes, the apparent brightnefs, 
difti'n&nefs, and magnitude, of an objeCt, are the only 
means by which our judgment is determined in eftimating 
the diftance of it. 
THEORY of REFRACTION. Plate II. 
Light, when proceeding from a luminous body, is 
invariably found to proceed in ftraight lines, without the 
lead deviation. But, if it happens to pafs obliquely from 
one medium to another, it always leaves the direction it 
had before, and affumes a new one; and this change of 
courfe is called its refrattion. After having taken this 
new diredtion, it then proceeds invariably in a ftraight 
line till it meets with a different medium, when it is again 
turned out of its courfe. It mull be obferved, however, 
that though, by this means, we may caufe the rays of light 
to make any number of angles in their courfe, it is im- 
poflible to make them defcribe a curve, except in one 
Angle cafe, namely, where they pafs through a medium 
the dentity of which uniformly either increafes or de- 
creafes. This is the cafe with the light of the celeftial 
bodies, which paftes downwards through dur atmofphere, 
and likewife with that which is reflected upwards through 
it by terreftrial objeCts. In both thefe cafes, it defcribes 
a curve of the hyperbolic kind ; but, at all other times,'it 
proceeds in ftraight lines, or in what may be taken for 
ftraight lines without any lenfible error. 
The phenomena of refraCtion are explained by an at¬ 
tractive power in the medium through which light paftes, 
in the following manner. All bodies being endowed with 
an attractive force, which is extended to fome diftance 
beyond their furfaces, when a ray of light paftes out of 
a rarer into a denfer medium (if this latter has a greater 
attractive force than the former, as is commonly the cafe), 
the ray, juft before its entrance, will begin to be attracted 
towards the denfer medium; and this attraction will con¬ 
tinue to aCt upon it till fome time after it has entered 
the medium ; and therefore, if a ray approaches a denfer 
medium in a direction perpendicular to its lurface, its 
velocity will be continually accelerated during its paifage 
through the fpace in which that attraction exerts itfelf 5 
and therefore, after it has parted that fpace, it will 
move on, till it arrive at the oppofite fide of the medium, 
with a greater degree of velocity than it had before it 
entered : fo that in this cafe its velocity only will be altered : 
whereas, if a ray enters a denfer medium obliquely, it 
will not only have its velocity augmented thereby, but 
its direElion will become lefs oblique to the furface ; juft 
as, when a ftone is thrown downwards obliquely from a 
precipice, it falls to the furface of the ground in a direc¬ 
tion nearer to a perpendicular one than that with which 
it was thrown from the hand. Hence we fee, that a ray 
of light, in parting out of a rarer into a denfer medium, 
is refracted towards the perpendicular; that is, fuppofmg 
a line drawn perpendicularly to the furface of the medium 
through the point where the ray enters, and extended 
both ways, the ray, in parting through the furface, is re¬ 
fracted or bent towards the perpendicular line ; or, which 
is the fame thing, the line, which it defcribes by its mo¬ 
tion after it has parted through the furface, makes a lefs 
angle with the perpendicular than the line which it de- 
fcribed before. Thefe pofitions may be illuftrated in the 
following manner. 
Let us fuppofe firft, that the ray paftes out of a vacuum 
into the denier medium ABCD, Plate II. fig. 1. and that 
the attractive force of each particle in the medium is ex¬ 
tended from its refpeCtive centre to a diftance equal to 
that which is between the lines AB and EF, or AB and 
GH; and let KL be the path defcribed by a ray of light 
in its progrefs towards the denfer medium. This ray, 
when it arrives at L, will enter the fphere of attraction of 
thofe particles which lie in AB, the furface of the denfer 
medium ; and will therefore ceafe to proceed any longer 
in the right line KLM, but will be diverted from its courfe 
by being attracted towards the line AB, and will begin 
to defcribe the curve LN, pafling through the furface AB 
in fome new direction, as OQ ; making a lefs angle with 
a line, PR, drawrj, perpendicularly through the point N, 
than it would have done, had it proceeded in its firft direc¬ 
tion, KLM. 
As we have fuppofed the attractive force of each particle 
to be extended-through a fpace equal to the diftance be¬ 
tween AB and EF, it is evident that the ray, after it has 
entered the furface, will ftill be attracted downwards, till 
it has arrived at the line EF ; for, till then, there will not 
be fo many particles above it which will attraCl it up¬ 
wards, as below that will attraCl it downwards. So that, 
after it has entered the lurface at N, in the direction of 
OQ, it will not proceed in that direction, but will con¬ 
tinue to defcribe a curve, as NS; after which it will pro¬ 
ceed ftraight on towards the oppofite fide of the medium, 
being attracted equally every way; and therefore will at 
laft proceed in the direction XST, ftill nearer the perpen¬ 
dicular PR than before. 
If 
