Nov. 1, 1865.] 



SCIENCE-GOSSIP. 



249 



COMPOUND EYES. 



I THINK if your correspondent, H. Yokes, 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 alto- 

 gether 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 many, and these not on a level surface, but on a 

 curve, or less spherical one. Now, let us look at 

 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 dis- 

 tance off, and you will see all as through a miniature 

 telescope ; or put a piece over some scales of butter- 

 flies, 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 conical 



^ ^ " /* 



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 but 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), drav/ 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 3, but is ab- 

 sorbed in the others before it reaches it : and so ou 

 with and p. In ni the light goes a little farther down 

 the tube but still is absorbed ere it reaches the retina. 

 So that, notwithstanding the multitude of lenses, 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 per- 

 plexed 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. - g, t. Scott. 



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 Vv^herewilh to 

 measure them. Galileo estimated the height of some 

 of the mountains to be nearly 29,000 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. Up- 

 wards 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 2'i/cho, are of tlie 

 cii'cular type. — Poptdar Science Revieio. 



