weather. The huge reflector is useful for occasional star-gazing, but the refracting telescope is the one always used for regular scientific observations; and the great aim of opticians is to improve the latter.
The length of Mr Clarke's telescope is small compared to the telescopes of former days. At the Observatory of Kew are the glasses of a telescope executed by Huygens. They belonged to the Philosophical Society, but the focal length being such as to require a tube as long as a tall church spire, they were never fitted up. Sometimes the glasses of such telescopes were used without a tube, the eye-glass being placed at an immense distance from the object-glass. The refracting telescope at once shrunk into small dimensions, at least in regard to length, as soon as the achromatic principle was applied by Dolland. The only way, previous to this, of obviating the defect of colour, was by making the focus very long; but Dolland, by simply making the object-glass compound, one part being of flint, the other of crownglass, dispensed with long tubes. An achromatic telescope of the present day, two feet in length, performs much better than one of Huygen's aerial telescopes 100 feet long. But while, for convenience, opticians try to make their telescopes as short as possible, they are struggling to expand in another direction, and to execute object-glasses as large as possible; and if three inches in advance of any other telescope be eflected, a great triumph will be gained.
