ORGANS OF PERCEPTION. 197 
is obvious, that the sources of error are numerous, and 
that the information thus gained is of little value, unless 
corrected by the experience of the sense of touch. 
As we judge of the magnitude of objects, by the angle 
which they form with the eye, so, in like manner, do we 
employ the same means to determine their Distance, when 
restricted to the organs of vision alone. But as bodies of 
the same dimensions, when placed at different distances, 
will form with the eye different angles, and as bodies of 
unequal dimensions may be so arranged, as to appear un- 
der the same angle, our notion of distance acquired by un- 
assisted vision, must likewise be very imperfect. It is true, 
that in judging of the distance of objects, their degree of 
illumination is likewise attended to. But as objects are not 
always luminous in proportion to their proximity, it may 
often happen that this character, instead of contributing to 
an accurate result, shall rather generate error. 
In judging of the condition of the Surface of any object, 
in order to.ascertain its inequalities by means of the eye, we 
attend to the degree of illumination of the different parts, 
and form our opinions from the manner in which the light 
and shade are distributed. But as the eye has not the 
power of perceiving the difference between a dark ground 
and a shaded place, nor between an illumined and a white 
spot, it is obvious, that, without the assistance of the sense 
of touch, it could not be determined whether the surface 
was rough or smooth, pitted or even. 
The eye assists us in determining the Motions of bodies ; 
‘but, like its intimations with regard to size and distance, it 
frequently misleads us. Thus, when an object is moving 
in.a straight line from us, it appears as if at rest. When 
sailing on a river in a boat, the objects which are stationary 
on the banks seem to be in motion, while the boat which is 
in Motion appears to be at rest. In like manner, the sun 
