The glass mirror having reached the degree of perfection desired, was then silvered on its concave surface by being plunged into a bath of nitrate of silver, dissolved in water, and mixed with certain proportions of gum galbanum, nitrate of ammonia, and oil of cloves. Half an hour in this bath was sufficient for the deposition of a film of silver of sufficient thickness to bear polishing. When finished, the mirror was found to 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. The substitution of a parabolic glass mirror for the ordinary metal speculum offers the triple advantage of greater lightness, increased distinctness, and more brilliant images. Fig. 48 represents the large silvered glass telescope constructed under M. Foucault’s direction for the observatory at Marseilles. It measures 32 inches in diameter, and has a focal length of a little more than 16 feet, and is put in motion by clockwork of a very perfect description, so that when once pointed at a star or planet it follows the object, which would otherwise disappear on account of the rotation of the earth. The path taken by the rays is precisely the same as in Newton’s telescope, the eye-piece being placed at the side of the tube, which is provided with a movable platform and staircase for the observer. [Illustration: FIG. 48.—Foucault’s Large Telescope. ] The optician to whose talent in his art this fine instrument is due, has recently executed several small telescopes upon the same model, at such a price as to bring them within the reach of amateurs with slender purses. The principal part of these telescopes, one of which is represented in fig. 49, (see next page), is the mirror, which is about 4 inches in diameter, and 24 inches’ focal length. The body, which is cylindrical, is made of brass, and revolves on two pivots placed horizontally at about one-third of its length from the bottom. The bearings on which the pivots move consist of two upright standards of metal, which are connected at the bottom, and revolve on a pin in the middle of the plate of the tripod stand. They are made of such a height that the lower portion of the instrument may pass between them, when it is necessary to observe objects in the zenith. By the turn of a screw the whole of the upper portion of the instrument may be dismounted and fixed on a lower standard, so that the observer may work sitting down if necessary. The body of the telescope is provided with a finder. One of the great advantages of this form of instrument is that it can be used for observations on the zenith without giving the observer those unpleasant cricks in the neck so inseparable from the use of ordinary telescopes in a nearly upright condition. The mirror will bear a power of 220 diameters, and shows the mountains of the moon, the phases of