IO

This film will assume the curve beneath it, and may be repolished many times, it being obviously impossible to touch the glass surface without having rubbed through the silver; but this need never occur, as when the film becomes thin or scratched away, it may be renewed at a trifling expense. It is, of course, very important that the glass surface itself be not much rubbed, as so exact is the method by which these mirrors are tested during their manufacture, that the very spot where this has been done for a few seconds could, on examination, be immediately pointed out. Another great advantage of a glass mirror over one of metal is the inferior weight of the former. In supporting a heavy metallic mirror, excessive care is required, not to have the slightest strain upon it; thus, with large instruments, the mirror often rests on an elaborate system of levers. It was found, with large metallic mirrors, that the presence of a single thread between the mirror and its supports very sensibly interfered with good definition. A glass mirror of the same diameter and thickness as one of metal will be found to weigh far less, and thus its mounting need not be so elaborate, and consequently not so expensive.

A further advantage will be at once discovered by an observer who, after working with a metal, turns his attention to a glass speculum of equal aperture; and that is the extra amount of light reflected from the silver. This is stated to be, on the authority of Sir J. Herschel, for Newtonian telescopes, as ’824 to *436. The light-grasping power of a glass speculum can be well ascertained by its performance on very faint stars, especially when they are close companions to brighter ones.

Since the introduction of silvered-glass mirrors they have been largely employed in physical observation, and where a large amount of light-grasping power is required, as in spectroscopic work.

Having already briefly described the various forms of telescopes, I will now consider each more fully and with special reference to the stands on which they are mounted, and to which they are individually or collectively suitable. To facilitate the description of the stands it will be useful to class all telescopes under two heads, which may be called, in respect to the position of the observer, Direct and Indirect. The former class includes all varieties of refracting telescopes; and of reflecting telescopes, the Gregorian and original form of Cassegrainian. The latter—Nasmyth's form of the Cassegrainian, the Herschelian, and Newtonian reflecting telescopes. • It will be as well, therefore, to follow the above order, describing the suitable stands after each class.

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