“But if a stone the gentle sea divide,
Swift rippling circles rush on every side,
And glimmering fragments of a broken sun,
Banks, trees, and skies in thick disorder run.”
And there is an end of the observations. If, however, the eyes can penetrate below the surface, the ruffling is of little consequence, so long as the water is clear. Consequently, whenever the top of the bank is sufficiently near the water, it is possible to continue the observations by lying down, and immersing the head above the eyes. This plan, however, is not a very comfortable one, although I have often followed it on a windy day when the surface was too ruffled to permit of vision in any other way.
Still, there is an instrument by which it is possible to counteract the ruffle of the surface, and to see objects with tolerable plainness. This is called the Water Telescope, and it is of very simple construction. Like the ordinary telescope, it consists of a tube, but, instead of the convex and concave lenses of that instrument, it has only a single glass at one end, and that glass is perfectly plane.
When used, the eye is applied to the open end, and the glazed end lowered into the water. The sight is then undisturbed by the ripple, and the effect is the same as if the eyes themselves were lowered beneath the surface.
It is much used in looking for shells, sea-urchins, and other creatures which live in the bed of the sea.
In the insect world we have an example of a natural Water Telescope. I do not say that the inventor of the Water Telescope took his idea from the insect, but the reader will see that he might very well have done so.
There are sundry little beetles popularly called Whirlwigs or Whirligigs, and scientifically known by the name of Gyrinus. All these names allude to the insect’s habit of whirling about on the surface of the water, with a movement which seems ceaseless and untiring. Allusion has already been made to the Whirlwigs on page [22].
Their object in their perpetual waltz is not so much amusement as food, which chiefly consists of the tiny insects which fall into the water. Now, in order to enable it to see both above and below the water, a peculiar structure is required. Generally the insects possess one pair of compound eyes, each group being set on the sides of the head. In the Gyrinus, however, there are two sets of these eyes, one pair being on the upper surface of the head, and the other on the lower surface. Thus, while it can use the upper pair for seeing objects which are out of the water, the lower pair of eyes, which are submerged, act the part of the Water Telescope, and enable it to see objects that are below the surface. Were it not for this precaution, even the ripples which it makes by its own rapid progress would prevent it from seeing.