existence, is the one now mounted in the Lick Observatory in California. It has an aperture of thirty-six inches in the clear. It was tested on seventy different nights, on veritable stars, before it was considered finished. It sometimes happens that important astronomical work is done with these temporarily mounted lenses, and some of them have quite a history. Thus, in ante-bellum days, an eighteen-inch objective was ground for the University of Mississippi, but, the war coming on, the lens was never sent for. It was afterward used at Chicago, and is now doing good work at Evanston, Ill. While it was temporarily mounted at Cambridgeport, Mr. Alvan G. Clark discovered the companion to the dogstar Sirius, a discovery for which he was awarded the Laland prize by the Imperial Academy of Paris. At the present time a twenty-inch objective for the University of Denver is in progress, while a forty-inch disk of crown glass awaits transformation into a lens for the University of Southern California. It was at first feared that such giant lenses would suffer injury by warping, but the experience at Mount Hamilton has been so reassuring that the present tendency is toward even larger glasses.
It would still be a difficult task, though a less difficult one than the present, if it were simply required to produce a perfect curvature, but the superiority of a lens depends upon its chemical composition as well as upon its geometric form. The problem may be summed up by stating that one must have as homogeneous a material as possible, to start with, and as symmetrical a form as the inequalities in the material will permit, to end with, the theoretical curves being in practice slightly modified to obviate any small irregularities in density.
The crown-glass lens with which the flint glass is combined in order to obtain achromatism is made in the same way, only that the material is a lime-soda silicate similar to that used in window glass instead of having lead and potash as its bases.
It would be easy to multiply illustrations of the use which science makes of the refractive power of glass, as in the stereopticon, the kaleidoscope, the camera, the projecting lantern, and in other apparatus of scientific or popular nature; but in the manufacture of all of them these two principles hold—the production of a heavy, uniform glass, and the shaping of this material into suitable form by processes of grinding and polishing.
There is, however, one application so important to philosophic thinking as to deserve special mention, even though it involves no new principle. What the astronomical telescope has been in the study of the physical features of the heavenly bodies, the glass prism is in the investigation of their chemical constitution. Had one spoken but a few years ago about the chemistry of the sun and stars, and seriously proposed their analysis, his hearers
