54 Transactions of the American Institute. 



through them. Yon have seen the representatives of the diiferent 

 large sized diamonds in the windows of the shops, and sometimes 

 how beantifnl thej look, each being formed of an exquisitely pure 

 little bit of glass. Kow, to form only one of these discs every part 

 must be as pure as that, and no one part of it must be more dense than 

 another. The mixture of the thing must be so accurate throughout, 

 it will be, as if you propose to your cook the problem of making » 

 cake from a given receipt, every crumb of which when analyzed by 

 the chemist should be found to be combined in the same nice way. 

 No wonder, then, that a mere disc of glass two feet in diameter or 

 more is the very last stretch of art at the present time, and that 

 the mere material of two such discs is worth in itself $3,500 in gold. 

 Then, when that is produced, you must shape the four surfaces 

 accurately, making the one destroy the color of the other with rigid 

 accuracy. That in a large telescope may be the work of two years 

 before it is accomplished. And yet that very difficult work is doing and 

 has been done. A few years ago a telescope with an aperture 

 of nine inches was the last result of the effort of art ; but they 

 have got up to a foot and fifteen inches ; and Mr. Fitch, 

 whose loss you have deplored here in New York, was wonderfully 

 successful in making these achromatic object glasses. Spencer, also, 

 and Eaton, and lastly Alvah Clark. We have among us in America one 

 of the best opticians in the world, Alvah Clark. He is just as 

 modest and unassuming as he is worthy ; and when I use the 

 last epithet to enhance the others, I say a very great deal. And I 

 say Princeton College honored herself when she gave him an A. M.. 

 not long ago. While that large telescope, which is in use in Cliicago^ 

 was getting ready, and before anybody had bought it, this persever- 

 ing, modest man turned his eye toward the heavens and discovered 

 the body that had disturbed the motions of Sirius, and the French 

 Academy therefore very properly gave him the gold medal for the- 

 best astronomical discovery of the 3'ear. Then, not only has the 

 telescope done this, but it has pictured things by the aid of photo- 

 graphy. The photograph of the moon by Rutherford, of JSTew York, 

 is one of the largest ever produced. Prof. Henry Draper, of New York 

 University, as I have said, has done the same thing. More than that, 

 photography has been brought in in a new and very curious way. 

 You have heard how long light is in coming from the stars. Starting 

 from one of those double stars, the shai-p-pointed, straight-traveling 

 pencil of a little beam, years in coming, disturbs the little molecular 

 aiTangement of a plate well prepared by the chemist, and writes 



