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82 THE FOUEFOLD BOOT. [CHAP. IV.
the vault of the sky : that is to say, for its appearing to have
greater horizontal than vertical extension. Both pheno
mena therefore are purely intellectual or cerebral, not optical.
If it be objected, that even when at its zenith, the moon
occasionally has a hazy appearance without seeming to be
larger, we answer, that neither does it in that case appear
red ; for its haziness proceeds from a greater density of
vapours, and is therefore of a different kind from that
which proceeds from atmospheric perspective. To this
may be added what I have already said: that we only
apply this mode of estimating distances in a horizontal,
not in a perpendicular, direction ; besides, in this case,
other correctives come into play. It is related of Saussure
that, when on the Mont Blanc, he saw so enormous a
moon rise, that, not recognising what it was, he fainted
with terror.
The properties of the telescope and magnifying glass,
on the other hand, depend upon a separate estimate
according to the visual angle alone : i.e., that of size
by distance, and of distance by size; because here the
four other supplementary means of estimating distances
are excluded. The telescope in reality magnifies objects,
while it only seems to bring them nearer ; because their
size being known to us empirically, we here account for
its apparent increase by a diminution of their distance
from us. A house seen through a telescope, for instance,
seems to be ten times nearer, not ten times larger, than
seen with the naked eye. The magnifying glass, on the
-contrary, does not really magnify, but merely enables
us to bring the object nearer to our eyes than would
otherwise be possible ; so that it only appears as large
as it would at that distance even without the magnify
ing glass. In fact, we are prevented from seeing objects
distinctly at less than from eight to ten inches distance
from our eyes, by the insufficient convexity of the ocular