CONSTRUCTION OF LARGE TP]LESCOPE LENSES.« 



By Dr. C. Faxtlhabeb. 



The three principal instruments for the study of the heavenly 

 bodies are the telescope, the spectroscope, and the photographic 

 camera; and since the two latter are made useful only as they are 

 attached to the former, it is the telescope Avhich we must still regard 

 as the key to imlock the doors of the universe. Readers have all 

 doul)tless seen a large telescope, and many have had an opportunity 

 of looking through one, for most observatories reserve certain hours 

 for the public. Accordingly a description of the instrument as a 

 whole may be omitted, and we will merely recall that, notwithstand- 

 ing it is so long and heavy, complicated mechanical and electrical 

 means are provided for pointing and accurately guiding the telescope, 

 so that it follows automatically the motion of any chosen celestial 

 object. But no less hard tluui the difficulty of providing these 

 mechanical adjuncts is the optical problem of providing the great 

 doul)le lens called the objective at the upper end of the tube. The 

 objective is the fundamental part of the telescoiK\ on whose excellence 

 the value of the whole instrument de]3ends, and not only its quality 

 but its size also is of the highest importance to make possible the 

 observation of objects otherwise forever invisible. Hence it is that 

 telescopes are designated, not by the maxinnun magnification which 

 they can produce, nor by their length, but rather by the diameter of 

 their objectives. Thus one speahs of the 40-inch of tlie ^'erkes 

 Observatory, the HO-inch of the Lick Observatory, and the ?>2-inch 

 Potsdam refractor. 



In order to study the constructicm of a great telescope objective, 

 the attention of the reader is now invited to a great optical glass 

 Avorks, of which there are but three principal ones in the world, 

 namely, those of Schott & Genossen, in Jena; Mantois, in Paris: and 

 Chance Bros. & Co., in Birmingham. 



"Translated, by permission, from Prometheus, Berlin, Vol. XV, Nos. 34-35, 

 1904. 



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