REFUTATION OF ARISTOTLE'S "VISUAL ANGLE" THEORY.
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?