XKAR AND DIST.VNT SIGHT. 
527 
shews how greatly our inferences respecting the distances of 
near objects, is governed by a perception of the degree of con- 
vergence of those axes. When, at the distance of sixteen 
feet, I cause two windows which are eight feet asunder, from 
centre to centre, wholly to coincide by squinting, the eyes be- 
come quiescent, and I see three windows, of which the middle 
one is most distinct, and seems smaller and nearer than the 
others. And so likewise, when I view the two straight legs 
of a pair of chemical tongs, held upright at seven inches dis- 
tance, and produce a coincidence by increasing the convergence 
of the optical axes, the middle of three legs, which then ap- 
pear, seems nearer than the others; and if I attempt to touch 
it by advancing a finger sideways, I find the finger arrive* at 
the point of convergence, or of single vision, at about three 
inches and a half distance from the eyes. But, on the con- 
trary, when the coincidence is produc' d by directing the sight 
to a remote object, the midille leg ; ppears much larger and 
more distant. This last experiment is less easy to be made than 
the other, because it is very unusual for us to pay attention to 
■ near object while the orgai s of perception are adjusted to a 
remoter one. Among the familiar instances in which the 
estimate of distance from the inclination of the optical axes 
comes into operation, may be mentioned the great difficulty 
of bringing two pens or pencils, or small rods, at once into 
contact, one being held in each hand, and the ends moved hori- 
eontally to each other, while one eye is conversed or kept 
•hut ; and the very great advantage to which [)ers{>ective paint- 
ings are viewed by a single eye, or through a tube or instru- 
ment which allows one eye only to be used. In either of these 
cases, the mind can derive no assistance from the position of 
the optical axes ; and in the latter the habitual judgment 
from the apparent magnitudes, situations, and tints of object, 
is permitted to give life and reality to the scene, without any 
controul from that position, which would tend to shew that th* 
whole lay in the same plane. — \V. N. 
ERRATA. 
