to the cause of the image, namely, the external object itself. In what
follows we shall deal with an observation which will show how right
Reid was in this respect.

Those familiar with this observation (well known indeed to those living
in the hilly and mountainous districts both here and on the Continent)
know that when distant features of the landscape, in an otherwise clear
and sunlit atmosphere, suddenly seem almost near enough to touch, rainy
weather is approaching. Likewise a conspicuous increase in distance,
while the sky is still overcast, foreshadows fine weather.

This effect (the customary 'explanation' of which is, as usual, of no
avail to us and so need not concern us here) ranks with phenomena
described in optics under the name of 'apparent optical depth', a
subject we shall discuss more fully in the next chapter. It suffices
here to state that it is the higher degree of humidity which, by
lending the atmosphere greater optical density (without changing its
clarity), makes distant objects seem to be closer to the eye, and vice
versa. (If we could substitute for the air a much lighter gas - say,
hydrogen - then the things we see through it would look farther off
than they ever do in our atmosphere.)

Observations such as these show us that (a) when external light strikes
the retina of our eye, our inner light is stimulated to move out of the
eye towards it; (b) in pressing outward, this inner light meets with a
certain resistance, and the extent of this determines at what distance
from the eye our visual ray comes to rest as the result of a kind of
exhaustion. Just as the outer light reaches an inner boundary at our
retina, so does the inner light meet with an outer boundary, set by the
optical density of the medium spread out before the eye, Outer and