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The object-glass of the telescope should be covered with an opaque cap, pierced by two circular holes about one-eighth of an inch in diameter and half an inch apart. The holes should be on opposite sides of the centre of the object-glass and equidistant from it, and the line joining the holes should be horizontal.
After considerable discussion as to the best form and principle of the proposed instrument the work was finally commenced. According to the calculations of the Observatory of Cambridge, the tube of the new reflector would require to be 280 feet in length, and the object-glass sixteen feet in diameter.
Ludwig accompanied her to the door of her apartments, bade her good night, and returned to the observatory. Already the disk of the moon was half obscured. Ludwig removed the astronomical eye-piece from the telescope, and inserted the tellurian glass instead; then he turned the object-glass toward the neighboring manor instead of toward the moon.
The next step was an even longer one, and it was again taken by a self-taught optician, Thomas Cooke, the son of a shoemaker at Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead ordered from him in 1863 a 25-inch object-glass.
He sees no parallelism of condition, then, by which chance could act in forming a crystalline lens, which answers to the condition of an optician's shop, where it might be possible in many ages for chance to combine existing forms into an achromatic object-glass.
Three horizontal and three vertical wires were used in the focus. These were illuminated by a speculum, near the object-glass, reflecting the light from a lantern placed over the axis, the upper part of the telescope-tube being partly cut away to admit the light. A divided circle, with pointer and reading microscope, was provided for reading the declination.
The astronomer, furnished with an eye-piece, and wishing to examine that image, must necessarily place himself beyond the point where the rays that form it have crossed each other; beyond, let us carefully remark, means farther off from the object-glass.
The first, constructed by Herschel, was 36 feet in length, and had an object-glass of 4 feet 6 inches; it magnified 6,000 times; the second, raised in Ireland, at Birrcastle, in Parsonstown Park, belonged to Lord Rosse; the length of its tube was 48 feet and the width of its mirror 6 feet; it magnified 6,400 times, and it had required an immense erection of masonry on which to place the apparatus necessary for working the instrument, which weighed 12-1/2 tons.
The anterior part of the tube is supported at both ends, and is thus fixed in a direction pointing towards the pole, with only the power of twisting axially. The posterior section is joined on to it at right angles, and presents the object-glass, accordingly, to the celestial equator, in the plane of which it revolves.
It is impossible to see these different images clearly at the same time, because each of them will render all the others indistinct. The achromatic object-glass, invented by Dollond, about 1750, obviates this difficulty, and brings all the rays to nearly the same focus.
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