eory. It is a matter of fact
that, if the mirror were perfectly vertical, only very acute vision
could detect the depression of the image of the sea-horizon below the
image of the eye-pupil. The depression can easily be calculated for any
given circumstances. Parallax encouraged observers to note very closely
the position of the eye-pupil in the image, so that most of them
approached the image within about ten inches, or the glass within about
five. Now, in such a case, for a height of one hundred feet above the
sea-level the image of the sea-horizon would be depressed below the
image of the eye-pupil by less than three hundredths of an inch--an
amount which could not be detected by one eye in a hundred. The average
diameter of the pupil itself is one-fifth of an inch, or about seven
times as great as the depression of the sea-horizon in the case
supposed. It would require very close observation and a good eye to
determine whether a horizontal line seen on either side of the head were
on the level of the centres of the eye-pupils, or lower by about
one-seventh of the breadth of either pupil.

The experiment is a pretty one, however, and well worth trying by any
one who lives near to the sea-shore and sea-cliffs. But there is a much
more effective experiment which can be much more easily tried--only it
is open to the disadvantage that it at once demolishes the argument of
our friend Parallax. It occurred to me while I was writing the above
paragraph. Let a very small mirror (it need not be larger than a
sixpence) be so suspended to a small support and so weighted that when
left to itself it hangs with its face perfectly vertical--an arrangement
which any competent optician will easily secure--and let a fine
horizontal line or several horizontal lines be marked on the mirror;
which, by the way, should be a metallic one, as its indications will
then be altogether more trustworthy. This mirror can be put into the
waistcoat pocket and conveniently carried to much greater height than
the mirror used by Parallax. Now, at some considerable height--say five
or six hundred feet above the sea-level, but a hundred or even fifty
will suffice--look into this small mirror while _facing_ the sea. The
true horizon will then be seen to be visibly below the centre of the
eye-pupil--visibly in this case because the horizontal line traced on
the mirror can be made to coincide with the sea-horizon exactly, and
will then be found _not_ to coi