Page:Hardwicke's Science-Gossip - Volume 1.pdf/265

I think if your correspondent, H. Vokes, comes to study the eyes of insects a little further, he will see that they are something far different from organs of touch, which, of all the strange ideas of the present day, seems to me to be passing strange. Paley teaches us that a piece of mechanism adapted to a certain purpose will show that its maker intended it to serve that purpose, and not an altogether different one. Now, let us trace the eyes of animals from one to another. Spiders, monoculi, crustacea, and insects. We shall find a likeness, though a difference. We shall find their organisation such as is adapted to visual organs; that amongst other things they are capable of refraction, like lenses, able in a beautiful manner to transmit light, and form correct images of objects—some moveable, some immoveable. Those that are moveable consist mostly of one or few lenses; the immoveable ones of nay, and these not on a level surface, but on a curve, or less spherical one. Now, let us look al the eye of a dragon-fly. It first consists of a cornea composed of some thousands of lenses, for their action shows them to be truly such. Put a piece of one under the microscope, and direct it to the windows, trees, or human beings, some little distance off, and you will see all as through a miniature telescope; or put a piece over some scales of butterflies, and you may make it act as a power of the instrument, that is, it will show the objects magnified. Now, from each of these lenses proceeds a tube lined with a dark pigment.

They are of a couical shape, as they must be, to converge regularly towards one point—the retina, or nervous centre. To go further into the mechanism of them would take up too much time and room; but in a rough way a section of a fly's eye will somewhat resemble the figure, in which a represents the cornea; b, the tube proceeding from it; and c, the retina. Now, we know how a single eye acts, and how it can be directed from one object to another; but in the eye of a fly, which is immovable, this cannot be done, and were there no means of preventing it, truly a fly would have nothing bat a confused vision—a multitude of images of the same object; but from the drawing we see how beautifully this is prevented, yet with a considerable extent of vision. Let x, y, o, p represent four objects opposite the four lenses of the cornea (a), draw a straight line of light from each object. That from x passes down the whole length of tube 1, but the light falls on the dark pigment in No. 2, &c., and is absorbed, and so in the other focals. The light from z, in like manner passes without interruption to the retina in 2, but is absorbed in the others before it reaches it: and so on with o and p. In m the light goes a little farther down the tube but still is absorbed ere it reaches the retina. So that, notwithstanding the multitude of leases, we see that no object is visible to the insect except that, the light from which can proceed without stoppage down the whole length of the tube. I should suppose the hairs on a fly's eye are something like our eyelashes, and not so many thousands of organs of touch. Try and catch a fly, watch it, and then see if it don't show most acute vision. Consider how little the mechanism of a fly's eye is fitted for touch, how well its lenses act, and we can hardly doubt but that light and not touch is intended by its Maker to act upon it; although we may not, with our present knowledge, just perceive the why and the wherefore it is formed as it is. Surely it is no wonder the poor dragon flies were troubled and perplexed to know what to do or where to go to with their eyes blinded. Cover any one's eyes, and put him into an open field, and see how utterly unable he is to guide himself straight, how confused he instantly becomes.

Mountains of the Moon.—It curiously enough happens that we were acquainted with the height of the lunar mountains before those on our own earth, or before the barometer was invented wherewith to measure them. Galileo estimated the height of some of the mountains to be nearly 20,000 feet high, and in recent times it has been found from actual measurement that one named Doerfel, at the southern part of the moon, is nearly 25,000 feet high. Upwards of twenty are higher than Mont Blanc. The highest peak on the earth is 28,180 feet; but if we compare the size of the earth and moon, we shall find that the proportion of the diameter of the latter to the height of its most elevated mountain is as 1 to 454, whilst the same ratio on the earth is as 1 to 1,481. Some of the highest of the mountains on the moon, as those of Newton and Tycho, are of the circular type.—Popular Science Review.