tube, open at the other end, the rays of light could be brought to a focus and directed to an eye-piece, where an observer, with his back to the object, it might be, could see it clearly and distinctly magnified. The mirror required to be of a parabolic form, and might be made of metal or of glass. Newton chose an alloy of tin and copper for the mirror or speculum, but he did not trouble himself about grinding it into the form of a parabola. The second reflecting telescope he made magnified thirty-eight diameters, and was presented to the Royal Society in 1671. Half a century passed before any farther step was taken with either refracting or reflecting telescope. Hadley, the inventor of the sextant, then took the matter up. In 1723 he made one on Newton's pattern, with a mirror of 6 inches aperture, and a focal length of 62 58 inches. Its eye-pieces magnified up to 230 diameters. A report on it was made to the Royal Society, of which the substance was that Newton's telescope "had lain neglected these fifty years," but Hadley had shown "that this noble invention does not consist in bare theory." Strange to say, in that very year an English gentleman had made a refracting telescope, which largely overcame the difficulties arising from colour. His was the first achromatic or colourless telescope: it remained the only one for another fifty years. Although its inventor lived all that time, he neither claimed first honours nor interfered with the patent of the second discoverer, Dollond.
Another half-century thus passed, and little or nothing had been done. Dollond had rediscovered in 1758 the method of counteracting colour in glass lenses, but no one seemed disposed to apply the prin-
