106 THE NEW WORLD OF SCIENCE 



more similar, whereby the shortening of the secondary spec- 

 trum could be effected. Boric acid is peculiar in lengthening 

 the red end of the spectrum, relative to the blue, while potas- 

 sium, and sodium have the opposite effect. In the old glass, 

 flint has a higher index and greater dispersion than crown 

 glass, and lengthens the blue more than the red. Hence it was 

 desirable to introduce into flint glass as large a percentage as 

 possible of boric acid. The work, being empirical, was very 

 tedious, but after a great many trials, in which the problems 

 of suitable crucibles, stirring apparatus, etc., were not incon- 

 siderable, a series of phosphates, borates, and boro-silicates was 

 successfully produced in small quantities. 



The question of annealing soon became important, and after 

 a great many trials and subsequent testings with polarized light, 

 the process known as fine annealing was perfected. It was 

 discovered that the temperature of solidification lay between 

 370 C, and 465 C, and by spreading the fall of 95 over an 

 interval of four weeks or more, perfect results were obtained. 

 This involved the construction of an oven with thermo regu- 

 lators, whereby the temperature might be kept at any point and 

 allowed to fall with any desired slowness. 



Up to 1886 the net result of all these epoch-making dis- 

 coveries and new processes was nineteen glasses of essentially 

 new optical characteristics, and the researches conducted by 

 Abbe and Schott, with, the help of the University and the 

 Prussian Diet, soon made Jena the world's center for the high- 

 est grade of optical glass. Through this small but essential 

 component of optical instruments of all kinds, Germany exer- 

 cised great power over scientific and military progress in optics. 

 The wide-spread use of German scientific instruments needs no 

 emphasis, and it is interesting to note that the best military 

 optics among the armies and navies of the Allies until long 

 after the war began, were made with Jena glass, and only the 

 fact that a large stock of this glass was in America enabled our 

 optical instrument-makers to carry on until American-made 

 optical glass could come to their relief. 



