CONSTRUCTIOlSr OF LARGE TELESCOPE LENSES. 169 



of tlie difficulty of separating large cenieiited lenses for sul)se(|uent 

 cleaning. 



After the lenses hav^ beeB placed in their cell there remains only 

 the final testing in the telescope tube itself. I shall not describe the 

 complicated centering apparatus employed in this test. The errors 

 of an objectiye and their causes are numerous, and their discoyery and 

 cori-ection demand great experience and skill. 



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

 liyes are located, and by whom they were made. In the fii-st place, 

 tliere is the objective made for the Paris Exposition of 11)00, but not 

 among the telescopes in ])resent use. It is 1.24 meters in diameter, 

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

 weighs MO 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, (~^al., with diameters, respectively, of 105 and 

 91 centimeters. Both Avere })oured at the Paris glass works and fig- 

 ured by Alvan Olark in Cambridgeport. Mass. 



They are both satisfactory, though not prepared entirely on the 

 basis of computation, but rather l)y 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 ol)jectiye was poured in Jena and figured at 

 the optical Avorks 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- 

 mony to the ability of (lernian manufacturers in this line. The Pots- 

 dam refractor has, in addition to the 80-centimeter ])h()tographic 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 telescoj^es are devised 

 solely for visual purposes, and can only be used for photography by 

 tlie aid of auxiliary lenses which 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 ()l)servatory of Nice, of efpial diameter, bv Henry Brothers, 

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

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



oThe Yerkes telescope is used as u i)hotograi)hic iustnniient by interposing; in 

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

 sensitive for the yellow rays. 



