Now, as these customary,
experimental means of suggesting distance do likewise suggest magnitude,
so they suggest the one as immediately as the other. I say they do not
(VIDE sect. 53) first suggest distance, and then leave the mind from
thence to infer or compute magnitude, jut suggest magnitude as
immediately and directly as they suggest distance.
78. This phenomenon of the horizontal moon is a clear instance of the
insufficiency of lines and angles for explaining the way wherein the mind
perceives and estimates the magnitude of outward objects. There is
nevertheless a use of computation by them in order to determine the
apparent magnitude of things, so far as they have a connexion with, and
are proportional to, those other ideas or perceptions which are the true
and immediate occasions that suggest to the mind the apparent magnitude
of things. But this in general may, I think, be observed concerning
mathematical computation in optics: that it can never be very precise and
exact since the judgments we make of the magnitude of external things do
often depend on several circumstances, which are not proportionable to,
or capable of being defined by, lines and angles.
79. From what has been said we may safely deduce this consequence; to
wit, that a man born blind and made to see would, at first opening of his
eyes, make a very different judgment of the magnitude of objects
intromitted by them from what others do.
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