human heart is equal to lifting two hundred and twenty tons one foot from the ground. What building-up does to strengthen the gun has been repeated in the case of the circular saw: driven at a high speed it becomes so highly heated at its periphery that the resulting expansion may crack the metal in pieces. In an improved method of manufacture the saw is hammered to a compression which gradually increases from rim to centre. In this way the tendency of the periphery to fly apart is withstood by the compressive forces at the central portion of the disc. This ingenious treatment of metal for guns and saws reminds us of a familiar resource in carpentry, illustrated on page 36. An ordinary book-shelf, if fairly long and not particularly stout, bends beneath its burden and may at last slip out from its mortices and fall with injury to its books. At the outset this is prevented by bending the shelf to convexity on its upper surface. Then a heavy load no more than brings the shelf to straightness, so that the books remain in their places with both safety and sightliness. Here a principle is involved worth a moment’s pause. An inventor asks, What effect will a working load exert which it is desirable to lessen or withstand? He gives his structure a form opposite to that which will result from an imposed burden, so that when at work his structure, a shelf, a cylinder, a saw, will assume its most effective shape. The Eye and the Dollond Lenses. From childhood we are familiar with the triangular prisms of glass which break a sunbeam into all the hues of the rainbow. A lens is a prism of circular form, and has, equally with an ordinary prism, the power to show rays of all colors. This was for a long time a source of error and annoyance in telescopic images. Sir Isaac Newton from some rough and ready experiments concluded that the trouble was beyond remedy, yet all the while his own eyeballs were transmitting images with little or no vexatious fringe of color. Let us note how Dollond set about a task which Newton deemed impossible. He knew, what Newton did not know, that crown glass disperses or scatters light only half as much as does flint glass, so he united a lens of the one to a lens of the other, and obtained a refracted or bent beam of light almost unchanged in its whiteness. Of course, in this combination there was an increased thickness of glass, but its doubled absorption and waste of light was a small drawback compared with the advantage of almost wholly excluding the tinted fringe which had so long vexed astronomers. In the eyeball are first a crystalline lens, next an aqueous humor, third a vitreous humor; these three so vary in their qualities of refraction and dispersion as to render images quite free from color fringes. Compound lenses on the Dollond principle, repeating the structure of an eyeball, are used in all good telescopes, microscopes, and cameras, and are now