All which seems to arise from want of a due application of our discerning
faculty, thereby to discriminate between the ideas that are in our
understandings, and consider them apart from each other; which would
preserve us from confounding those that are different, and make us see
what ideas do, and what do not include or imply this or that other idea.

67. There is a celebrated phenomenon, the solution whereof I shall
attempt to give by the principles that have been laid down, in reference
to the manner wherein we apprehend by sight the magnitude of objects. The
apparent magnitude of the moon when placed in the horizon is much greater
than when it is in the meridian, though the angle under which the
diameter of the moon is seen be not observed greater in the former case
than in the latter: and the horizontal moon doth not constantly appear of
the same bigness, but at some times seemeth far greater than at others.

68. Now in order to explain the reason of the moon's appearing greater
than ordinary in the horizon, it must be observed that the particles
which compose our atmosphere intercept the rays of light proceeding from
any object to the eye; and by how much the greater is the portion of
atmosphere interjacent between the object and the eye, by so much the
more are the rays intercepted; and by consequence the appearance of the
object rendered more faint, every object appearing more vigorous or more
faint in proportion as it sendeth more or fewer rays into the eye. Now
between the eye and the moon, when situated in the horizon, there lies a
far greater quantity of atmosphere than there does when the moon is in
the meridian. Whence it comes to pass that the appearance of the
horizontal moon is fainter, and therefore by sect. 56 it should be
thought bigger in that situation than in the meridian, or in any other
elevation above the horizon.