540 THE POPULAR SCIENCE MONTHLY. 



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 be- 

 sides a tendency to the positive or the research of applications imme- 

 diately 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 di- 

 rect ratio with the aperture. According to Leon 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 mir- 

 rors of silvered glass in which Leon Foucault attained so great per- 

 fection 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 inconsider- 

 able work ; the experience at Melbourne affords an illustration of this 

 fact. It is then with good reason that telescopes of the Foucault sys- 

 tem are preferred in France. Finally, to sum up every thing, the fu- 

 ture is perhaps not for great mirrors, but for great object-glasses. In- 

 deed, with an equal aperture, a refracting telescope furnished with a 

 good object-glass far surpasses a reflecting telescope ; the great re- 

 fractors 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 Observa- 

 tory 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 intrust- 

 ed 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 



