VIS 
witli otlier optical investigations, he pub- 
lished in liis Optical Lectures,- first printed 
in 1674. According to him, we refer every 
point of an object to the place from which 
tile pencils of light issue, or from which 
they would have issued, if no reflecting or 
refracting substance intervened. Pursuing 
this principle, Dr. Barrow proceeded to in- 
vestigate the place in which the rays issu- 
ing from each of the points of an object, 
and that reach the eye after one reflection 
or refraction, meet; and he found that 
when the refracting surface was plane, and 
the refraction was made from a denser me- 
dium into a rarer, those rays would always 
meet in a place between the eye and a per- 
pendicular to the point of incidence. If a 
convex mirror be used, the case will be the 
same ; but if the mirror be plane, the rays 
will meet in the perpendicular, and beyond 
it, if it be concave. He also determined, 
according to these principles, what form 
the image of a right line will take when it 
is presented in different manners to a sphe- 
rical mirror, or when it is seen through a 
refracting medium. 
M. Bouguer adopts Barrow’s general 
maxim, in supposing that we refer objects 
to the place from which the pencils of rays 
seemingly converge at their entrance into 
the pupil. But when rays issue from below 
the surface of a vessel of water, or any 
other refracting medium, ,he finds that 
there are always two different places of 
this seeming convergence : one of them of 
the rays that issue from it in the same ver- 
tical circle, and therefore fall with different 
degrees of obliquity upon the surface of 
the refracting medium ; arid another of 
those that fall upon' the surface with the 
same degree of obliquity, entering the eye 
laterally with respect to one another. He 
says, sometimes o'tfe of these images is at- 
tended to by the mind, and sometimes the 
other ; and different images may be observ- 
ed by different persons. And lie adds, that 
an object plunged in wafer affords an ex- 
ample of this duplicity of images. 
From the principle above illustrated, se- 
veral remarkable phenomena of vision may 
be accounted for : as — That if the distance 
between two visible objects be an angle 
that is insensible, the distant bodies will ap- 
pear as if contiguous; whence, a con- 
timious. body being tlie result of several 
contiguous ones, if the distances between 
several visibles subtend insensible angles, 
they vvill appear one continuous bocly ; 
wliich gives a pretty illustration of the no- 
tion of a continuum. Hence also parallel 
VIS 
lines, and long vistas, consisting of parallel 
rows of trees, seem to converge more and 
more the further tliey are extended frqpi 
tlie eye; and the roofs and floors of long 
extended alleys seen, the former to descend, 
and the latter to ascend, and approacii 
each other; because the apparent magni- 
tudes of their perpendicular intervals are 
perpetually diminisidng, while at the same, 
time we mistake their distance. See 
Priestley’s Light and Colours. 
The mind perceives the distance of visi- 
ble objects, 1st, From tlie different configu- 
rations of tire eye, and the manner in 
which the rays strike the eye, and in wliich 
the image is impressed upon it. For the 
eye disposes itself differently, according to 
the different distances it is to see ; viz. for 
remote objects the pupil is dilated, and the 
crystalline brongiit nearer the retina, and 
. the whole eye is made more globous; on 
the contrary, for near objects, the pupil is 
contracted, the crystalline thrust forwards, 
and the eye lengthened. Again, the dis- 
tance of visible objects is judged of by the 
angle the object makes; from the distinct 
or confused representation of the objects ; 
and from the briskness or feebleness, or the 
rarity or density of the rays. To this it is 
owing, 1st, Tiiat objects which appear ob- 
scure or confused, are judged to be more 
remote; a principle which the painters 
make use of to cause some of their figures 
to appear further distant than others on the 
same plane. 2d, To this it is likewise ow- 
ing, that rooms whose walls are whitened, 
appear the smaller; that fields covered with 
snow, or white flowers, appears less than 
when clothed with grass ; that mountains 
covered witli snow, in the night time, ap- 
pear the nearer, and that opaque bodies ap- 
pear the more remote in the twilight. 
The magnitude of visible objects, is 
known chiefly by the angle contained be- 
tween two rays drawn from the two ex- 
tremes of the object to the centre of tlie 
eye. An object appears so large as is tl:e 
angle it subtends ; or bodies seen under a 
greater angle, appear greater; and those 
under a less angle, less, &c. Hence the 
same thing appears greater or less as it is 
nearer the eye or further off. And tliis is 
called the apparent magnitude. But to 
judge of the real magnitude of an object, 
we must consider tlie distance ; for since a 
near and a remote object may appear un- 
der equal angles, though the magnitudes be 
different, tiie distance must necessarily ba 
estimated, because the magnitude is great 
or small according as the distance is. So 
