beyond is plainly visible, so,
standing in that submarine shadow, all around is dark, though beyond
the sable curtain of the shadow the view is clear. Apply this optical
fact to the ghastly story of a diver's alleged experience in the
cabin of a sunken ship. It is narrated that there was revealed to
his appalled sight the spectacle of the drowned passengers in various
attitudes of alarm or devotion when the dreadful suffocation came.
The story is told with great effect and power, but unless a voltaic
lantern is included in the stage furniture, the ghastly tableaux must
sink into the limbo of incredibilities.

The cabin of a sunken vessel is dark beyond any supernal conception of
darkness. Even a cabin window does not alter this law, though it
may be itself visible, with objects on its surface, as in a child's
magic-lantern. As the rays of light pass through an object flatwise,
like the blade of a knife through the leaves of a book, and may be
admitted through another of like character in the plane of the first,
so a ray of light can penetrate with deflection through air and water.
But becoming polarized, the interposition of a third medium ordinarily
transparent will stop it altogether. Hence the plate-glass window
under water admits no light into the interior of a cabin. The distrust
of sight grows with the diver's experience. The eye brings its habit
of estimating proportion and distance from an attenuated atmosphere
into another and denser medium, and the seer is continually deceived
by the change. He hesitates, halts, and is observant of the pitfalls
about him. A gang-plank slightly above the surface of the deck is
bordered, where its shadow falls, by dismal trenches. There is a range
of hills crossing the deck before him. As he approaches he estimates
the difficulty of the ascent. At its apparent foot he reaches to
clamber the steep sides, and the sierra is still a step beyond his
reach. Drawing still nearer, he prepares to crawl up; his hand touches
the top; it is less than shoulder-high.

But perhaps the strongest illustration of the differing densities
of these two media is furnished by an attempt to drive a nail
under water. By an absolute law such an effort, if guided by sight
independent of calculation, must fail. Habit and experience, tested
in atmospheric light, will control the muscles, and direct the blow
at the very point where the nail-head is not. For this reason the
ingenious expedient of a voltaic