is a long and tedious operation, requiring many trials and repetitions of the process.
The object-glass has but two pieces, — a plano-concave lens of flint and a double convex lens of crown glass.
The Washington equatorial (shown in Plate LXIX.) has a clear aperture of 26 inches and a focal length of 31 feet 6 inches, its total length being 32 feet 6 inches. The rough glass for the object-lens was received by Messrs. Clark in December, 1871, and was ground, polished, and finished in November, 1872.
Another year was required to finish the tube and complete the other parts of the instrument.
The tube is of thin steel, in three pieces, and is mounted upon a pillar of brick supported by an arched foundation of bluestone, and capped by a block of sandstone weighing about two tons.
The dome inclosing the instrument is 41 feet in diameter and 25 feet high.
It rests upon a tower of equal diameter and 21 feet in hight.
For lightness, and in order that the temperature may be maintain