io8 HERSCHEL AND HIS WORK 



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 Koyal Society in 1671. Half a century passed 

 before any farther step was taken with either refract- 

 ing 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 62f inches. Its 

 eye-pieces magnified up to 230 diameters. A report on 

 it was made to the Royal Society, of which the sub- 

 stance 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 gentle- 

 man 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- 



