FRAUNHOFER 199 



which his master did not possess. Fraunhofer used part of 

 the present to buy his freedom from his master, and with 

 the other half he bought a glass grinding machine. He also 

 learned engraving on metal, in the hope of being now able 

 to make himself independent. But he was not successful 

 in earning his living, so that he could find nothing better than 

 to return to his former employment. Five years later he was 

 taken into a larger optical works, which he himself then 

 helped to become famous. He soon improved all the 

 machines, but also most particularly the manufacture of 

 glass. His efforts led to success in making large pieces of 

 flint glass (a lead glass with high refraction and dispersion) 

 free from striae, and also in other respects suitable for the 

 finest optical purposes; and he also introduced an exact con- 

 trol over the refractive index of the glasses used, by measur- 

 ing the co-efficient of refraction (according to Snell's law) by 

 means of prisms cut from the glasses to be tested. 



These efforts led Fraunhofer to two very great successes. 

 The first was the discovery of 'Fraunhofer's lines' in the solar 

 spectrum, which Newton had not seen; the other was the 

 possibility of providing astronomy with much more effective 

 and larger refracting telescopes, particularly for the purposes 

 of measurement. We will discuss these two achievements 

 both separately and in their connection. 



Fraunhofer's good glass enabled him to produce prisms 

 giving much purer spectra than Newton had been able to 

 obtain. Furthermore, he fully recognised the other re- 

 quirements necessary to obtain pure spectra: use of a fine 

 slit for admission of the light, parallelism of the rays in the 

 prism, and then along with these, observation through a 

 telescope. The result was immediately - as so often in the 

 case of a clean experiment - a new discovery. He saw in the 

 spectrum of the sun the dark lines named after him, and he 

 proved by exact measurement of the position of these lines 

 with all kinds of variations in the experimental conditions, 



