Memoir of the Life (fM- Fraurihofer. 5 



siderable dimensions which came from the furnaces of Bene- 

 dictbauern. These lenses were destined for the instruments of 

 the observatory of Buda. It was afterwards agreed to trans- 

 fer all the optical part of the establishment to Benedictbauern, 

 and to give the complete direction of it to Fraunhofer. Our 

 philosopher had already studied catoptrics, and had even writ- 

 ten a Memoir on the aberration which takes place without the 

 axis in reflecting telescopes. He showed that hyperbolic mir- 

 rors are preferable to parabolic ones, and he also communicat- 

 ed the invention of a machine for polishing hyperbolic surfaces. 

 He now, however, resolved to give up this branch of the sub- 

 ject, as his time was fully occupied in the preparation of lenses. 



One of the most difficult problems in practical optics is to 

 give to spherical surfaces the last polish with that degree of 

 exactness which theory requires, because this final operation 

 destroys in part that form which had been previously given to 

 the surfaces. M. Fraunhofer succeeded in remedying this 

 evil by a machine which not only did not injure the fine sur- 

 face obtained by grinding, but which actually corrected the 

 irregularities committed in the first operation. It has also the 

 advantage of making the result independent of the skill of the 

 workman. 



In examining the glass which he used in reference to the 

 undulations and striae which it contains, he found that, in the 

 flint glass manufactured at Benedictbauern, there was often 

 not a single piece free of those irregularities which disperse and 

 refract the light falsely. Pieces of the same melting had not 

 even the same refracting power, and this was perhaps more 

 common in the Enghsh and French flint glass. After obtain- 

 ing these results, Fraunhofer reconstructed the furnaces, pro- 

 cured the necessary instruments, and took the direction of all 

 the meltings. 



He had learned from experience, that flint glass could be 

 made so that a piece at the bottom of the pot had exactly the 

 same refractive power as a piece from the top ; but his success 

 was of short duration, for the succeeding meltings showed that 

 this was merely accidental. Undaunted, however, by failure, 

 he recommenced his experiments, in which he always melted 

 four quintals at once, and after long and severe labours, he 



