out of focus, as if he
looked, in air, through a pair of powerful spectacles that were utterly
unsuited to him. He cannot distinguish the letters of the largest print
in a newspaper advertisement; he cannot see the spaces between the
outstretched fingers, at arm's length, in clear water; nor at a few
inches' distance in water that is somewhat opaque. I read a short paper
on this subject, at the British Association in 1865, in which I showed
the precise cause of this imperfection of vision and how it might be
remedied. If the front of our eyeballs had been flat, we should have had
the power of seeing under water as clearly as in air; but instead of
being flat, they are very convex, consequently our eye stamps a concave
lens of high power into the water, and it is the seeing through this
concave eyeglass which our eyeball makes for itself, that causes the
indistinctness of our vision. Knowing the curvature of the eyeball, it is
easy to calculate (as I did in the memoir mentioned above) the curvature
of a convex lens of flint-glass that should, when plunged into water,
produce effects of an exactly equal and contrary value, exactly
neutralizing the effects of the concave eyeglass of water, if it were
held immediately in front of the pupil of the eye. I have made several
experiments with a view to obtaining serviceable spectacles, for seeing
under water. The result is as follows:--experience has shown the distance
from the eyeball at which spectacle-glasses can be most conveniently
placed; now at that distance, the joint effect of the concave water-lens
and the convex glass spectacle-lens, is to produce an opera-glass of
exceedingly low magnifying power, that requires a small adjustment for
accurate definition at different distances.

If the spectacle-lens be of flint-glass and doubly convex, each of its
faces should have a curvature of not greater than 6 1/2 tenths of an
inch, nor more than 8 1/2 tenths of an inch in radius: within these
limits, it is practicable to obtain perfectly distinct vision under water
by pressing the spectacles forwards or backwards to a moderate degree.
Lenses of these high magnifying powers are sometimes sold by
spectacle-makers, for persons who have undergone an operation for
cataract. I have tried, but hitherto without much success, to arrange the
fittings by which the lenses are secured so that by a movement of the jaw
or by an elevation of the eyebrows, I could give the necessary adjustment