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He now zealously devoted himself to the improvement of the achromatic telescope; and, after a prolonged study of the theory of lenses, and many toilsome experiments in the manufacture of flint-glass, he succeeded in perfecting, December 12, 1817, an object-glass of exquisite quality and finish, 9-1/2 inches in diameter, and of 14 feet focal length.
And even after Chester More Hall in 1729, and John Dollond in 1757, had shown that chromatic aberration could be nearly eliminated by the combination of a flint-glass lens with one of crown glass, William Herschel, who began his observations in 1774, devoted his skill entirely to the making of reflectors, seeing no prospect of much advance in the power of refractors.
Nearly every one interested in the subject is aware that this object-glass is composed of two lenses a concave one of flint-glass and a convex one of crown-glass, the latter being on the side towards the object. This is the one vital part of the telescope, the construction of which involves the greatest difficulty.
Cicerone here pointed to other iron vessels, in shape like the bowl out of which the giant Blunderbore ate his bread and milk, while trembling little Jack peeped at him from the oven; but these bowls were filled with a beautiful scarlet powder of fine consistency. "That is red-lead, one of the most important ingredients in fine flint-glass, as it gives it brilliancy and ductility.
In the time of Dollond it was found scarcely possible to make good disks of flint-glass more than three or four inches in diameter. Early in the present century, Guinand, of Switzerland, invented a process by which disks of much larger size could be produced.
Pound and Bradley used one presented to the Royal Society by Hadley in 1723. Hawksbee, Bradley, and Molyneaux made some. But James Short, of Edinburgh, made many excellent Gregorian reflectors from 1732 till his death in 1768. He soon acquired a great reputation for his telescopes of moderate size; but there was a difficulty in making flint-glass lenses of large size.
Knives, forks, and house cloths are often deficient: these accidents might be obviated, if an article at the head of every list required the former to be produced whole or broken, and the marked part of the linen, though all the others should be worn out. Glass is another article that requires care, though a tolerable price is given for broken flint-glass.
1400 pounds sand, 350 " ash, 100 " soda, 800 " red-lead, and about 100 " cullet. This was to be a fine quality of flint-glass, and to it might be added coloring-matter of any desired tint; but in the choice and proportion of this lay one of the principal secrets of the art.
Reflectors are brilliant engines of discovery, but they lend themselves with difficulty to the prosaic work of measuring right ascensions and polar distances. A signal improvement in the art of making and working flint-glass thus most opportunely coincided with the rise of a German school of scientific mechanicians, to furnish the instrumental means needed for the reform which was at hand.
Take a piece of flint-glass, beat it to a fine powder, and grind it well with the white of an egg, and it joins china without riveting, so that no art can break it in the same place. You are to observe, that the composition is to be ground extremely fine.
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