But observation warns the spectator of the delusive character of all that splendor of color.  He lifts a box from the ooze:  he appears to have uncorked the world.  The hold is a bottomless chasm.  Every indentation, every acclivity that casts a shadow, gives the impression of that soundless depth.  The bottom of the sea seems loopholed with cavities that pierce the solid globe and the dark abysses of space beyond.  The diver is surrounded by pitfalls, real and imaginary.  There is no graduation.  The shallow concave of a hand-basin is as the shadow of the bottomless well.

If the exploration takes place in the delta of a great river, the light is affected by the various densities of the double refracting media.  At the proper depth one can see clearly the line where these two meet, clean cut and as sharply defined as the bottom of a green glass tumbler through the pure water it contains.  The salt brine or gelatinous sea-water sinks weighted to the bottom, and over it flows the fresh river-water.  If the latter is darkened with sediment, it obscures the silent depths with a heavy, gloomy cloud.  In seasons of freshet this becomes a total darkness.

But even on a bright, sunshiny day, under clear water, the shadow of any object in the sea is unlike any shade in the upper atmosphere.  It draws a black curtain over everything under it, completely obscuring it.  Nor is this peculiarity lost when the explorer enters the shadow; but, as one looking into a tunnel from without can see nothing therein, though the open country 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