ived unreserved
acceptance. The one which usually figures in text-books is that we
unconsciously compare the sun and moon, when low down in the sky, with
the terrestrial objects in the same field of view, and are therefore
inclined to exaggerate the size of these orbs. Some persons, on the
other hand, imagine the illusion to have its source in the structure of
the human eye; while others, again, put it down to the atmosphere,
maintaining that the celestial objects in question _loom_ large in the
thickened air near the horizon, in the same way that they do when viewed
through fog or mist.

The writer[14] ventures, however, to think that the illusion has its
origin in our notion of the shape of the celestial vault. One would be
inclined, indeed, to suppose that this vault ought to appear to us as
the half of a hollow sphere; but he maintains that it does not so
appear, as a consequence of the manner in which the eyes of men are set
quite close together in their heads. If one looks, for instance, high up
in the sky, the horizon cannot come within the field of view, and so
there is nothing to make one think that the expanse then gazed upon is
other than quite _flat_--let us say like the ceiling of a room. But, as
the eyes are lowered, a portion of the _encircling_ horizon comes
gradually into the field of view, and the region of the sky then gazed
upon tends in consequence to assume a _hollowed-out_ form. From this it
would seem that our idea of the shape of the celestial vault is, that it
is _flattened down over our heads and hollowed out all around in the
neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a
consequence of their very great distance, all the objects in the heavens
necessarily appear to us to move as if they were placed on the
background of the vault; the result being that the mind is obliged to
conceive them as expanded or contracted, in its unconscious attempts to
make them always fill their due proportion of space in the various parts
of this abnormally shaped sky.

From such considerations the writer concludes that the apparent
enlargement in question is merely the natural consequence of the idea we
have of the shape of the celestial vault--an idea gradually built up in
childhood, to become later on what is called "second nature." And in
support of this contention, he would point to the fact that the
enlargement is not by any means confined to the sun and moon, but is
every whit as marked in