2. Some[51] hold that distant objects seem to us lesser only because
they are seen under a smaller visual angle. Elsewhere[52] we have shown
that this is wrong; and here we shall limit ourselves to the following
considerations. The assertion that a distant object seems less because
it is perceived under a smaller visual angle supposes that the rest
of the eye still sees something outside of this object, whether this
be some other object, or something external, such as the air. But if
we suppose that the eye sees nothing outside of this object, whether
this object, as would a great mountain, occupy the whole extent of the
glance, and permit nothing beyond it to be seen; or whether it even
extend beyond the sweep of the glance on both sides, then this object
should not, as it actually does, seem smaller than it really is, even
though it fill the whole extension of the glance. The truth of this
observation can be verified by a mere glance at the sky. Not in a
single glance can the whole hemisphere be perceived, for the glance
could not be extended widely enough to embrace so vast an expanse. Even
if we grant the possibility of this, and that the whole glance embraces
the whole hemisphere; still the real magnitude of the heaven is greater
than its apparent magnitude. How then by the diminution of the visual
angle could we explain the smallness of the apparent magnitude of the
sky, on the hypothesis that it is the diminution of the visual angle
which makes distant objects appear smaller?