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. THE IRIS OF THE EYE. [Illustration: IRIS OF HUMAN EVE.] [Illustration: COTTERILL’S LOCK.] I have often wondered, when contemplating the astonishing mechanism by which the Iris of the Eye is able to contract or enlarge the pupil according to the amount of light, whether any similar mechanism would be used in Art. As anatomists know, the Iris is composed of two layers. One consists of radiating fibres, which serve to enlarge the pupil, while the other layer surrounds the latter, and by its elasticity serves to contract it. As any one may see by looking in a mirror and shifting the light, the pupil is perpetually changing its diameter, but always retaining its circular shape. A glance at the illustration will show the two layers, and aid the reader in understanding the mode in which they work. Some years ago, while looking at the account given by Mr. J. Price of a lock invented by Mr. Cotterill, I saw at once that the inventor, whether consciously or not, had followed the mechanism of the eye, as far as metal could be expected to imitate animal fibre. In the very centre of the lock there is a small circular opening, resembling the pupil of the eye, and serving to admit the key, just as the pupil admits light. Around this pupil, if we may so call it, are ranged some twenty thin steel slides which move in channels, up and down which they slide. Round the circumference of the lock are a corresponding number of spiral springs, each of which presses on the base of a slide, and forces it towards the centre. The reader will now see that the radiating slides of the lock represent the radiating fibres of the iris, and that the spiral springs represent the circular fibres. Both perform the same office, the steel slides regulating the size of the aperture, and the spiral springs pressing them all towards the centre. The key of the lock answers the same purpose as does light in the eye, which by its mysterious pressure enlarges or contracts the pupil.