69. Farther, the air being variously impregnated, sometimes more and
sometimes less, with vapours and exhalations fitted to retund and
intercept the rays of light, it follows that the appearance of the
horizontal moon hath not always an equal faintness, and by consequence
that luminary, though in the very same situation, is at one time judged
greater than at another.

70. That we have here given the true account of the phenomena of the
horizontal moon will, I suppose, be farther evident to anyone from the
following considerations. FIRST, it is plain that which in this case
suggests the idea of greater magnitude must be something which is itself
perceived; for that which is unperceived cannot suggest to our perception
any other thing. SECONDLY, it must be something that does not constantly
remain the same, but is subject to some change or variation, since the
appearance of the horizontal moon varies, being at one time greater than
at another. And yet, THIRDLY, it cannot be the visible figure or
magnitude, since that remains the same, or is rather lesser, by how much
the moon is nearer to the horizon. It remains therefore that the true
cause is that affection or alteration of the visible appearance which
proceeds from the greater paucity of rays arriving at the eye, and which
I term FAINTNESS: since this answers all the forementioned conditions,
and I am not conscious of any other perception that doth.

71. Add to this that in misty weather it is a common observation that the
appearance of the horizontal moon is far larger than usual, which greatly
conspires with and strengthens our opinion. Neither would it prove in the
least irreconcilable with what we have said, if the horizontal moon
should chance sometimes to seem enlarged beyond its usual extent, even in
more serene weather. For we must not only have regard to the mist which
happens to be in the place where we stand; we ought also to take into our