“Lastly, the spiders called Errantes, or Wanderers, have their eyes still more scattered, the lateral ones being placed at the margin of the cephalothorax.”
Yet, although each eye produces a separate image, it is clear that upon the mind of the Spider only a single idea can be impressed, for that otherwise all would be confusion. There must, therefore, be some mechanism in the structure of the eye, the nature of which we are not as yet able to understand.
A still more remarkable instance of a natural Multiplying-glass may be found in the eyes of many insects.
The form of multiplying-glass shown in the accompanying illustration is probably familiar to most of my readers. It consists of a convex piece of glass, cut into a number of facets, and showing in each facet a distinct and separate image of the object to which it is directed. Now, the compound eyes of insects are constructed on much the same principle, except that the number of facets is infinitely more. Taking, for example, the eyes of the Tortoise-shell Butterfly, we find that there are about seventy thousand lenses or facets. Now, it is possible, with care, to remove the eye from the insect, cleanse it, and arrange it in ä microscope in such a way that objects can be seen through it. When this is done, a separate image is seen in each facet, just as is the case with the Multiplying-glass, only, as the facets are very much more numerous, the effect is proportionately more striking.
The reader may notice that the facets of the insect eye appear to be hexagons as perfect as those of the honey-comb. This appearance is probably due to the fact that each eye is covered with a convex plate of glassy brightness and transparency, and that, when such objects are viewed from the front, they appear to have hexagonal instead of rounded outlines. A familiar example of this fact may be found in the glass tumblers which are ornamented with rounded projections on their surface. If a photograph of one of these tumblers be taken, the resemblance to the hexagonal markings of the insect eye is so close that the tumbler might easily be taken for the eye.
OPTICS.
CHAPTER II.
THE WATER TELESCOPE.—IRIS OF THE EYE.—MAGIC LANTERN.—THE SPECTROSCOPE.—THE THAUMATROPE.
Limits to Sight in the Water.—Effect of a Ripple.—The Eyes under Water.—The Water Telescope, its Structure and Mode of Use.—Gyrinus, or Whirlwig-beetle, and its Double Set of Eyes.—The Iris of the Eye, and its Double Set of Contractile Fibres.—Cotterill’s Lock and its Structure.—The Magic Lantern and its Principle.—Chinese Shadows.—Spectre of the Brocken.—An Adventure in Wiltshire.—Effect of the Halo.—The Spectroscope.—Its Structure explained.—A Star on fire.—Motes in the Sunbeams.—Bessemer Steel made by aid of the Spectroscope.—Absorption Bands.—Detection of Blood.—A Man’s Life saved by the Spectroscope.—The Pocket Spectroscope.—The Rainbow, Dewdrop, Soap-bubble, Opal, and Pearl.—The Thaumatrope.—Structure of the Retina.—Complementary Colours.—The Zoetrope and Chromatrope.—Wheel Animalcules and their Structure.—An Optical Delusion.
The Water Telescope.
EVERY one who has watched the movements of the various creatures which live below the surface of the water is aware how entirely dependent he is on the unruffled character of that surface. No matter how clear the water may be, the least ruffling of the surface will effectually shut out all sight:—