to be very agreeable to the principles we have laid down, and
receives a most easy and natural explication from them. For the further
clearing' up of this point it is to be observed that what we immediately
and properly see are only lights and colours in sundry situations and
shades and degrees of faintness and clearness, confusion and
distinctness. All which visible objects are only in the mind, nor do they
suggest ought external, whether distance or magnitude, otherwise than by
habitual connexion as words do things. We are also to remark that, beside
the straining of the eyes, and beside the vivid and faint, the distinct
and confused appearances (which, bearing some proportion to lines and
angles, have been substituted instead of them in the foregoing part of
this treatise), there are other means which suggest both distance and
magnitude; particularly the situation of visible points of objects, as
upper or lower; the one suggesting a farther distance and greater
magnitude, the other a nearer distance and lesser magnitude: all which is
an effect only of custom and experience; there being really nothing
intermediate in the line of distance between the uppermost and lowermost,
which are both equidistant, or rather at no distance from the eye, as
there is also nothing in upper or lower, which by necessary connexion
should suggest greater or lesser magnitude. 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 seve