FRAUNHOFER 20i 



The measurement of the index by means of his Hnes, 

 and the manufacture of glasses better adapted in their proper- 

 ties than those hitherto usual, and free from striae, was 

 Fraunhofer's means for the construction of large telescopes 

 exceeding all hitherto constructed, which he was able to 

 supply to astronomy. This method has also since remained 

 the standard, and has continually produced further success in 

 technical optics. Huygens' telescope, with which he dis- 

 covered the rings and moons of Saturn, had, like Galileo's, 

 only simple lenses; Newton had preferred to make use of 

 reflecting telescopes, which were free from colour eff"ects, 

 and thenceforth reflecting telescopes of increasing size had 

 been used for penetrating more and more deeply into the 

 depths of cosmic space. 



Since Fraunhofer's time, the refracting telescope again 

 acquired increasing importance, to which his very much im- 

 proved mechanical construction as regards the mounting and 

 measuring apparatus greatly contributed. By means of a 

 Fraunhofer telescope supplied to the observatory in Konigs- 

 berg, success was for the first time gained in the discovery of 

 the parallax of a fixed star, which had been sought in vain 

 since the time of Copernicus, and concerning which Bradley 

 had made such great endeavours, more than a hundred 

 years previously. Star No. 6i in the constellation Cygni 

 was the first to show such a displacement every half year 

 superimposed on its own proper motion (and the aberration); 

 it only amounted to 0.3 seconds of arc, from which the dis- 

 tance of the star is easily calculated as being ten light years. 

 The second star also for which a parallax was found, Vega 

 (distant twenty-seven light years) was measured by means of 

 a Fraunhofer telescope; this was the Dorpat refractor, which 

 at that time was regarded as a giant telescope,^ being four 



^ Compare in this connection, Josef Fraunhofer und sein optisches 

 Institut, by A. Seitz, Berlin, 1926, pp. 78 ff. The diameter of lenses has 

 now increased to about four times this. 



