Page:Popular Science Monthly Volume 6.djvu/558

540 in seeing that varied activity displayed in the numerous observatories of Great Britain, and including in its sphere every branch of celestial science, must it not be granted that something else is developed besides a tendency to the positive or the research of applications immediately useful?

It is worthy of note that the efforts of amateurs are particularly directed toward the realization of instruments of unusual dimensions, destined to sound the depths of the firmament. But the construction of mirrors or object-glasses of very great diameter is of fundamental interest for the progress of physical astronomy. Not only is the brightness of images proportioned to the aperture, that is, to the diameter of the instrument, but the optical power, or the faculty of separating two luminous points closely united, increases also in a direct ratio with the aperture. According to Léon Foucault, an aperture of at least thirty-nine inches is required to distinguish two points from each other whose apparent distance is equal to the tenth of a second of an arc. The two Herschels, Lord Rosse, Mr. Lassell, finally the commission that had charge of the construction of the Melbourne telescope, gave the preference to telescopes with a metallic mirror; is this preference justifiable? The question admits of doubt. The mirrors of silvered glass in which Léon Foucault attained so great perfection reflect a larger portion of light than metal mirrors; according to the experience of Mr. Wolf, a telescope with a silvered mirror reflects 80 per cent, of the incident light, while with metallic mirrors only 40 per cent, can be utilized. Besides, glass mirrors are lighter, and it is easy to silver them anew when the surface is tarnished. Metal mirrors need to be frequently repolished, which is no inconsiderable work; the experience at Melbourne affords an illustration of this fact. It is then with good reason that telescopes of the Foucault system are preferred in France. Finally, to sum up every thing, the future is perhaps not for great mirrors, but for great object-glasses. Indeed, with an equal aperture, a refracting telescope furnished with a good object-glass far surpasses a reflecting telescope; the great refractors of Dorpat and Pultowa rival reflectors of a double or triple diameter. We have already seen that a refractor of twenty-five inches aperture has been completed by Messrs. Cooke. The Observatory of Paris has possessed since 1855 a disk of flint-glass and one of crown-glass whose dimensions are sufficient to make an object-glass of nearly thirty inches in diameter, and in 1868 the Government voted $80,000 for the construction of a refractor which should be furnished with this object-glass and for that of a reflector with nearly four feet aperture. The mirror of the reflector, the work of which was intrusted to Mr. Martin, is almost finished; the cutting of the object-glass will be immediately commenced. This will be the most powerful glass that has yet been undertaken, and this time it is France that will take the precedence of other nations. It is hoped that this will