CONSTRUOTTON OF LARGK TKLKSCOPE LENSES. 169 



of (lio (lifliciilty of s('|):u':i(in<2: l:ii-<;c cemented lenses for subsecinent 

 c'leanin<;. 



After the lenses have been plaeed in their cell there remains only 

 the firial testin»2: in the telescope tnbe itself. I shall not describe the 

 complicated centering apparatns employed in this test. The errors 

 of an objective and their causes are numerous, and their discovery and 

 (.'ori-ection demand great exi)erience and skill. 



In conclusion, we may inquire where the telescopes of largest objec- 

 tives are located, and b}^ whom they were made. In the first place, 

 there is the objective made for the l^aris Exposition of 1000, but not 

 among the telescopes in present use. It is 1.21- meters in diameter, 

 and the glass alone weighs 580 kilograms, of which the convex lens 

 weighs 300 and the concave lens '2'20 kilograms. The cost of the two 

 lenses was 75,000 francs. These disks were poured by Mantois and 

 ground by Martins, both of Paris. Up to the present time the objec- 

 tive has not been usefully employed. The second and third places, 

 as regards size alone, are taken by the objectives of the Yerkes 

 Observatory, near Chicago (1897), and that of the Lick Observatory, 

 at Mount Hamilton, C"al., with diameters, respectively, of 105 and 

 91 centimeters. Botli were }>oured at the Paris glass works and fig- 

 ured by Alvan Clark in Cambridgeport, Mass. 



They are both satisfactory, though not prei)ared entirely on the 

 basis of computation, but rather by repeated trials, and brought to 

 their completion by the so-called method of local correction. After 

 them in size comes the great refractor of the Potsdam Observatory, 

 prepared solely for celestial photography and having a diameter of 

 80 centimeters. This objective was poured in Jena and figured at 

 the optical works of C. A. Steinheil S()hne, in Munich, in 1899. It is 

 recognized to be of the highest order of merit and is a strong testi- 

 jnony to the ability of German manufacturers in this line. The Pots- 

 dam refractor has, in addition to the 80-centimeter photographic lens, 

 a second visual lens of 50 centimeters diameter, and being thus a 

 double refractor is perhaps the largest astronomical instrument in 

 use in the world. Both of the great American telescopes are devised 

 solely for visual purposes, and can onl}'^ be used for photography by 

 tlie aid of auxiliary lenses Avhich cut off some of the light." 



Among other large objectives may be enumerated the Pulkova 

 refractor, at St. Petersburg, by Clark, diameter 70 centimeters; objec- 

 tive of the Observatory of Nice, of equal diameter, by Henry Brothers, 

 of Paris; the objective of the Vienna Observatory, of 71 centimeters 

 aperture, by Martins, and the Treptower objective, of 70 centimeters 



a The Yerkes telescope is used jis a ])hot()grai)hif' instninipnt by interposing in 

 front of the plate a color screen for removing the violet rays and exposing plates 

 sensitive for the yellow rays. 



