FRAUNHOFER 203 



consisted of fine wires, stretched in large numbers exactly 

 parallel to one another, and at exactly equal distances apart; 

 he then scratched gratings on glass, coated with gold leaf, by 

 means of a dividing engine, which removed the gold in paral- 

 lel lines, and finally, he engraved plain glass with a very fine 

 diamond point. The finest grating which he made in this 

 manner had 300 lines to the millimetre. The finer the 

 grating, the longer the spectra which it gives. Fraunhofer 

 again found his dark lines in grating spectra of the sun's 

 light - a further proof of the purity of the spectra, and also of 

 the strict relation of these lines to the light of the sun. Both 

 the grating and the prism are used to-day for spectral in- 

 vestigations; the first has the advantage, on account of the 

 peculiarity we have mentioned, of allowing measurements of 

 wave-length to be made with ease and very great exactness, 

 and Fraunhofer was also the first to carry out such measure- 

 ments with an accuracy, for example as regards his lines, 

 which had hitherto appeared quite unattainable. 



Almost a century after Fraunhofer's time it was discovered 

 that the newly-found and extremely short ether' waves 

 (X-rays), could be investigated by means of gratings; a grating 

 of suitable fineness was provided by crystals, which have 

 their atoms arranged at equal distances apart (X-ray spectro- 

 scopy). Fraunhofer scarcely allowed himself any rest in his 

 scientific work, and particularly in the work of his optical 

 factory, and even when he fell ill in 1825, with inflammation 

 of the lungs, he did not take a holiday. He was not 

 married. He lived a very modest and simple life; but the 

 Munich Academy paid him great honour. He did not re- 

 cover from the disease; he died when only thirty-nine years 

 of age. His grave in Munich bears the epitaph Approxima- 

 vit sidera (He brought the stars nearer). One of his bio- 

 graphers^ praises in him, summarising excellently what we 

 see from his work and his life, 'profound perspicacity, 



^ E. Lommel, Gesammelte Schriften von Josef Fraunhofer , Munich, 1888. 



