CONSTKUCTION OF LARGP: TELESCOrE LENSES." 



By Dr. C. Faulhaber. 



The tliroo 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 which we must still regard 

 as the key to unlock the doors of the imiverse. Readers have all 

 doubtless 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 moti(m of any chosen celestial 

 object. But no less hard than the difliculty of providing these 

 mechanical adjuncts is the optical problem of providing the great 

 double lens called the objective at the upper end of the tube. The 

 objective is the fundamental part of the telescope, on wdiose excellence 

 the value of the whole instrument depends, 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 maximum magnification which 

 tliev can i)roduce, nor b}' their length, but rather by the diameter of 

 their objectives. Thus one speaks of the 40-inch of the Yerkes 

 Observatory, the 3C-inch of the Lick Observatory, and the 32-inch 

 l^otsdam refractor. 



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

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

 works, 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, 



1004. 



163 



