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THE TELESCOPE.
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reflect 92 per cent. of the light incident on its surface, the loss in the case of achromatic object-glasses and metal specula being 20 and 35 per cent. respectively.
Fig. 48.—Foucault's Large Telescope.
The substitution of a parabolic glass mirror for the ordinary metal speculum offers the triple advantage of greater lightness, increased distinctness, and more
