458 
OBSERVATIONS ON SOUNDNESS. 
much more closely. This want of convergence between the 
axes of the two eyes has the further effect of causing the pic¬ 
tures upon the two retinae to be nearly identical, and, con¬ 
sequently, the idea of projection is not so strongly excited ; 
nor are we able to distinguish with the same certainty be¬ 
tween a well-painted picture in which the lights and shades 
are preserved, and the objects themselves in relief. 
Our notion of the size of an object is closely connected with 
that of its distance. It is founded upon the dimensions of the 
picture projected on the retina, and the dimensions of this 
picture will vary according to the laws of optics inversely as 
the distance;—being, for example, twice as great when the 
object is viewed at the distance of one foot as when it is 
carried to the distance of two feet. AYhen we know the rela¬ 
tive distances of two objects, the estimation of their real 
comparative sizes from their apparent size is easily effected by 
a simple process of mind; but this is not the case, when we 
only guess at their distances, and our estimate of the size of 
objects, even moderately remote, is as much effected by states 
of the atmosphere as that of their distance, the one being, in 
fact, proportional to the other. Thus a slight mist, which 
gives the idea of increased distance, will also augment the 
apparent size; because in order that an object two miles off 
should produce a picture upon the retina of the same extent 
with that made by an object one mile off, it must have double 
the dimensions. It is evident that our perception of the 
size of objects must be acquired by experience, in the same 
manner as that of their distance has been shown to be. 
We have now to consider briefly some other phenomena 
of vision, in W'hich the acts of the mind that have been just 
alluded to do not participate. The contraction of the pupil, 
under the stimulus of light, seems to be effected by a sphinc¬ 
ter muscle, which surrounds the orifice, and which is put in 
action by a branch of the third pair of nerves. This is an 
action with w'hich the will has nothing to do, and it takes 
place entirely without our consciousness- Although it is due 
to the stimulus of light, yet there is reason to believe that 
the consciousness of the presence of light is not requisite, 
and that it is, therefore, a purely reflex action. The optic 
nerve seems to be the channel through w’hich the impression 
is conveyed to the nervous centre; whilst the third pair is 
that through which the motor impulse is conveyed to the 
iris: but there is some ground for the idea that the fifth pair 
may in some degree convey the requisite stimulus when the 
optic nerve has been divided. That the dilatation of the pupil 
is a muscular action, appears probable from the fact that the 
