GL A SS-MAKING. 6 2 1 



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, 111. While it was temporarily mounted at Cambridge- 

 port, Mr. Alvan G. Clark discovered the companion to the dog- 

 star 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 chemi- 

 cal composition as well as upon its geometric form. The problem 

 may be summed up by stating that one must have as homogene- 

 ous 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 ob- 

 viate 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 sci- 

 ence makes of the refractive power of glass, as in the stereopti- 

 con, the kaleidoscope, the camera, the projecting lantern, and in 

 other apparatus of scientific or popular nature ; but in the manu- 

 facture of all of them these two principles hold the j)roduction 

 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 



