300 Telescopes — l^^fa of Fraunhofer. 



perhaps more common in the English and French flint glass. 

 After obtaining these results, Fraunhofer reconstructed the 

 furnaces, procured the necessary instruments, and took the 

 ^direction of all the meltings. 



lie 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, how- 

 ever, by failure, he recommenced his experiments, in which 

 he always melted four quintals at once, and after long and 

 severe labors, he discovered the numerous causes which oc- 

 casioned his want of success. 



As the English crown glass had many undulations and im- 

 purities, Fraunhofer resolved to manufacture it also. Diffi- 

 culties of a new kind here presented themselves, so that he 

 did not partly succeed til! after a whole year's labor. He 

 found also, that with whatever degree of accuracy he follow- 

 ed the theory in the construction of achromatic object-glas- 

 ses, his expectations were never realized. On the one hand, 

 he was convinced that it was wrong to neglect certain quan- 

 tities, such as the thickness of the lens and the higher pow- 

 ers of the apertures, merely to obtain commodious formulae ; 

 and on the other hand, there was no exact method for deter- 

 mining the exponents of refraction and dispersion in the 

 glass, used for achromatic object-glasses. The first of these 

 inconveniences he avoided by a new method, in which he 

 neglected no quantity upon which the required degree of ex- 

 actness depended. Hitherto, achromatic object glasses had 

 only been calculated for rays proceeding from a point in the 

 axis of the lens, but Fraunhofer considered the deviations 

 from all points situated without the axis, and this is always a 

 minimum in his object-glasses. In this consists principally 

 the difference between ids glasses and those made in Eng- 

 land. 



The difficulty liitherto experienced in determining the re- 

 fractive and dispersive powers of bodies, arises chiefly from 

 the circumstance that the spectrum has no definite termina- 

 tion, and that the passage from one color to another was so 

 gradual, and indistinctly marked, that in large spectra the an- 

 gles could not be measured with a greater accuracy than 

 from ten to fifteen minutes. In order to avoid this inconven- 

 ience, Fraunhnrrrsnfceeded, by a very ingenious contrivance, 

 in obtaining homogeneous light of each color in the spec- 



