to get rid of this inconvenience was the first problem presented to the mind of Herschel. The second was how to grind mirrors of such shape and size as to allow the application of magnifying powers enormously greater than any which had hitherto been considered possible. He succeeded in both attempts. The first difficulty was surmounted in a manner so simple that one is astonished it should have eluded the great inventors who preceded him. It occurred to him that if, instead of placing his reflector at right angles to the axis of his telescope, he inclined it a little forward, the image would be focused at a point on the edge of the tube, he could then dispense altogether with the second mirror and with the aperture in the reflector, and direct his eyepiece directly on the principal mirror itself. It was a case of Columbus and his egg over again.
Sir William summoned his brother Alexander from Hanover, and after Easter, when the termination of the Bath season left them a little leisure, they began to construct a telescope eighteen or twenty feet long! Every room was turned into a workshop. In the drawing-room worked a cabinet-maker, constructing tubes and stands for telescopes; in another room a huge turning-machine was erected, which Alexander picked up in Bristol.
Every leisure moment was eagerly snatched at for resuming some work which was in progress, without taking time for changing dress. Miss Herschel complains that they were continually tearing their lace ruffles, or bespattering them with molten pitch. On the grass-plot behind the house preparations were made for erecting the twenty-feet telescope, the precursor of that giant instrument which was afterwards the glory of Slough. The grinding of specula used formerly to be performed by hand, no machinery being sufficiently exact. The tool on which they were shaped was turned into the required form, and covered with coarse emery and water; the specula were then ground on it to the necessary figure, and afterwards polished with putty, or oxide of tin. To grind a speculum six or eight inches in diameter was considered a work of great labour;[1] what then must have been the difficulties incurred by the Herschels, who undertook to grind specula four feet in diameter? Miss Herschel was constantly in attendance on her brother while the grinding was going on.[2] She used to read to him while he was engaged in polishing. The authors selected were generally the "Arabian Nights," or the novels of Sterne and Fielding. She, however, managed to spare time for "two lessons a week" from Miss Fleming, the celebrated dancing-mistress, "to drill me for a gentlewoman; God knows how she succeeded!" In the midst of these multifarious occupations she mentions having copied the scores of "The Messiah" and "Judas Maccabeus" into parts for an orchestra of nearly one hundred performers, and the vocal parts of "Samson," besides instructing the treble singers, of whom she was now herself the first. William and Alexander Herschel used to throw themselves into their work with a glee like that of schoolboys out for a holiday. One Saturday night the brothers returned about twelve o'clock from a concert, pleasing themselves all the way with the idea that they would be at liberty to spend the next day, except a few hours' attendance at chapel, altogether at the turning-bench. Not liking to scandalize the good people of Bath by grinding their tools on Sunday, they ran out with a lantern to their landlord's grindstone, and set to work on their delicate task in semi-obscurity. They would probably have worked till daylight, but William was brought back fainting with the loss of one of his finger-nails. We ought, perhaps, to apologize for dwelling on these trifling details. Our excuse is that they make us know a great man better.
Pending the completion of the great telescope, the brothers manufactured several of smaller dimensions. Sir William had one of five, and one of seven feet focal length.
On the evening of the 13th of March, 1781, Herschel was engaged in examining some small stars in the vicinity of the constellation Gemini, when his attention was attracted to one more than the rest. He applied to his telescope higher magnifying powers, and found, to his surprise, that the apparent diameter of the body increased considerably. It was not, then, a fixed star, for no magnifying power presents one of those distant luminaries as other than a point of light. Careful ex-
- ↑ Lord Rosse's telescopes.
- ↑ "Je lis le passage suivant dans un mémoire de Lalande, imprimé en 1783 et faisant partie de la préface du tombe v'ii des 'Ephémérides des mouvements célestes.'"
"Chaque fois qu'Herschel entreprend de polir un miroir (de télescope), il en a pour dix, douze, quatorze heures d'un travail continu. Ii ne quitte pas un instant même pour manger, et reçoit des mains de sa sœur les aliments sans lesquels on ne pourrait supporter une si longue fatigue: pour rien au monde Herschel n'abandonnerait son travail; suivant iui, ce serait le gâter."
