166 CONSTRUCTION OF LARGE TELESCOPE LENSES. 



none among these wliich would answer the jiurpose the process would 

 then be repeated. AVhen a rough block of suitable size and quality 

 was obtained it was put in a crucible of about the proper lens form. 

 The whole was then again melted and cooled and then polished for 

 testing. As an example of the cost in time spent in this procedure, it 

 will be recalled that the Paris glass works required four years for the 

 {production of the tAvo lenses of the 3G-inch Lick objective. The 

 melting was done twenty times, and each time a month was spent in 

 the cooling. On the other hand, the Jena glass works employing the 

 improved processes, prepared both disks of the slight!}" smaller Pots- 

 dam 80-centimeter objective in a few months. 



It maybe of interest to rehearse briefly the story of the rapid devel- 

 opment of the industry of optical glass making in Germany, prin- 

 cipally during the last ten years. 



The pioneer in the production of glass for astronomical purposes, 

 according to purely scientific methods, was the renowned Joseph 

 von Fraunhofer, of Munich (1787-1826). But it is only twenty 

 years since Professor Abbe and the glass manufacturer. Doctor 

 Schott, of Jena, took up the work where Fraunhofer laid it down, 

 and succeeded in replacing the old flint and crown glasses by new 

 varieties of glass, by means of which the chromatic differences of 

 spherical aberration are nearly eliminated. The production of the 

 new glasses on a commercial scale began in the autumn of 1884. In 

 order to support the very costly preliminary experiments, the Prus- 

 sian Government made considerable grants of monev^ in consideration 

 of the national value of the work. This governmental support was 

 required but two years, for the undertaking progressed favorably 

 and the productions found recognition almost inmiediately in the 

 whole optical world, so that soon not only (jcrman, but foreign 

 optical establishments, placed most of their orders for material in 

 Jena. Not only are the common crown and flint glasses made here, 

 but also a great number of imin-oved crown and flint glasses, con- 

 taining boric and phosphoric acids, to diminish the secondary sjjec- 

 truni on the one hand, and on the other containing metallic oxides, by 

 means of which the dispersion and refraction may be increased or 

 diminished. An extensive exhibition of these products was witnessed 

 by the visitors who attended the Berlin Gewerbe-Ausstellung in 189(), 

 There were shown disks for the construction of telescopic objectives 

 of 110 and 125 centimeters diameter, and these Avere the largest pieces 

 of optical glass which had then been made. Not only is optical glass 

 ])roduced for all kinds of instruments of precision, but also there is3 

 made at Jena glass tubing for physical, chemical, manufacturing, 

 and medicinal purposes, and all sorts of chemical glassware, such as 

 flasks, beakers, and retorts, besides cylinders for gas and petroleum 



