WATER-SURFACE PLANTS AND ANIMALS. 235
thin clean glass rod, which will be surrounded by an ascending
capillary curve where it penetrates the surface-film, it is often
possible to push about small bodies on the surface without actually
touching them, thus showing that the capillary curves produced
by the said bodies are of the opposite kind, namely descending
ones, in accordance with the rule of repulsion between dissimilar
curves. The experiment can evidently also be made with a thin
rod made of or coated with some water-repellent material, when
the attractions and repulsions will of course be reversed.
But the prettiest way of demonstrating the capillary curves
and their nature is to observe their shadows when the objects
on the surface are placed in a shallow white dish and viewed
in sunlight or under a strong artificial light. Looked at in this
way a Pond-skater will be found to throw a shadow in which
the legs appear to terminate in large expansions, each pair of
characteristic shape due to the varying length of leg placed
upon the surface, as already mentioned. As each dark expansion
is moreover surrounded by a narrow margin of bright light it
is obviously produced by a capillary depression acting as a
concave lens. In the case of ascending capillary curves, the
shadows thrown have bright centres showing that they act as
convex lenses. I have seen a little beetle-larva creeping along
the underside of the surface-film and every time one of its legs
touched the surface, a minute spot of light appeared on the
bottom of the dish, proving that the leg did not realty break
through the surface-film, but only pushed it up into an ascending
curve. I think it would have been difficult to have demonstrated
this fact by any other method.
Yet another means of investigating surface-film phenomena
in relation to at least the smaller water-surface organisms will,
I hope, be found in the form of aquarium microscope which I
brought before the Royal Microscopical Society last year. With
this instrument, which may have the lower part of the body-tube
and the objective immersed in the water, it is possible to use the
objective horizontally half in and half out of the water and so
to bring the surface-film itself into focus, with any objects which
may be in its immediate vicinity. The objective can also be
pointed directly upwards so that the underside of the surface-
film and the undersides of any objects floating upon it may be
examined from below. Very little work has as yet been done