VIS 
tiiat the real magnitude is in the compound 
ratio of the distance and the apparent mag- 
nitude ; at least when the subtended angle, 
or apparent magnitude, is very small; other- 
wise, the real magnitude will be in a ratio 
compounded of the distance and the sine of 
the apparent magnitude, nearly, or nearer 
still its tangent. Hence, objects seen un- 
der the same angle, have their magnitudes 
in the same ratio as their distances. The 
cliord of an arc of a circle appears of equal 
magnitude from evety point in the circimi- 
■'^'erence, though one point be vastly nearer 
than another. Or if the eye be fixed in 
any point in the circumference, and a right 
line be moved round so as its extremes be 
always in the periphery, it will appear of 
the same magnitude in every position. And 
the reason is, because the angle it subtends 
is always of the same magnitude. And 
hence also, the eye being placed in any an- 
gle of a regular polygon, the sides of it 
will all appear of equal magnitude ; being 
all equal chords of a circle described about 
it. If tlte magnitude of an object directly 
opposite to the ey^e be equal to its distance 
from the eye, the wliole object will be dis- 
tinctly seen, or taken in by tlie eye, but 
nothing more. And the nearer you ap- 
proach an object, the less part you see of 
it. The least- angle under which an ordi- 
nary object becomes visible, is about 0116 
minute of a degree. 
The figure of visible objects is estimated 
chiefly from our opinion of the situation of 
the several parts of the object. Tliis opi- 
nion of the situation, &c. enables the mind 
to apprehend an external object under tliis 
or that figure, more justly than any simili- 
tude of tlie images in the retina, with tlie 
object cap; the images being often ellipti- 
cal, oblong, &c, when the objedts they ex- 
hibit to the mind are circles, or squares, &c. 
Tiie laws of vision, with regard to the fi- 
gures of visible objects, are. 3. That if 
tlie centre of the eye be exactly in the dri 
rection of a rigiit line, the line will appear 
only as a point. 3. If the eye be placed 
in the direction of a surface, it will appear 
only as a line. 3. If a body be opposed 
directly towards the eye, so as only one 
plane of the smface can radiate on it, the 
body will appear as a surface, 4. A remote 
arch, viewed by an eye in the same plane 
with it, will appear as a right line. 5. A 
sphere, viewed at a distance, appears a cir- 
cle. 6. Angular figures, at a distance, ap- 
pear loiind. 7. If the eye look obliquely 
m the centre of a regular figure, or a cir- 
VIS 
cle, the true figure will not be seen ; but 
the circle will appear oval, &c. 
VISION, is the act of seeing or of per- 
ceiving external objects by the organ of 
sight. As every point of an object, ABC, 
(Plate XVI. Miscel. fig. 11.), sends out 
rays in all directions, some rays from every 
point on the side next the eye, will fall upon 
the cornea, between E and F, and by pass- 
ing on through the humours and pupil of 
the eye, they will be converged to as many 
points on the retina, or bottom of the eye, 
and will thereon form a distinct inverted 
picture, eba, of the object. Tims the pen- 
cil of rays, g^r s, that Hows from the point, 
A, of the object, will be converged to the 
point, a, on the retina ; those from the 
point, B, will be converged to the point b ; 
those from the point, C, will be converged 
to the point, c; and so of all the interme- 
diate points, by which means the whole 
image, a be, is formed, and the object made 
visible, although it must be owned that the 
ihethod by which the sensation is carried 
from the eye by the optic nerve to the com- 
mon sensory in the brain, and there dis- 
cerned, is above the reach of our concep- 
tion. Tliat vision is effected in tliis manner 
may be demonstrated experimentally. Take 
a bullock’s eye, w'hile it is fresh, and hav- 
ing cut off the tliree coats from the back 
part, quite to the vitreous humour, put a 
piece of wliile paper over that part, and 
hold the eye towards any bright object, 
and you will see an invei'ted picture of the 
object upon the paper. Tlie diariieters of 
images at the bottom of the eye are propor- 
tional to the angles which the objects sub- 
tend at the eye, the same as in a lens, and 
are reciprocally as the distances of the same 
object viewed in different places. The eye 
is in reality no more than a camera obsCura, 
for the rays of light flowing from all the 
points of an object, through the pupil of the 
eye, do by the refraction of its humours, 
paint the image thereof in the bottom of 
the eye : just so it is in the camera obscura, 
where all the rays refracted by a lens in the 
window-shutter, or passing through a small 
hole in it, paint the image on the opposite 
wall. Some properties of the eye are these : 
the eye can only see a very small part of an 
object distinctly at once. For the collate- 
ral parts of an object are not represented 
distinctly in the eye ; and therefore the eye 
is forced to turn itself successively to the 
several parts of the object it wants to view, 
that they may fall near the axis of the eye, 
where alone distinct vision is performed. 
