308 Telescopes—Life of Fraunhofer. 
perhaps more common in the English and French flint glass. 
After obtaining these results, Fraanhofer reconstructed the 
furnaces, procured the necessary instruments, and took the 
direction of all the meltings. 
e 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 meJted four quintals at once, and after long and 
severe labors, he discovered the numerous causés 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 till after a whole year’s labor. He 
found also, that with whatever degree of accuracy he follow- 
ed the theory in the construction of achromatie 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 formule ; 
and on the other hand, there was no exact method for détér- 
minimum in his object-glasses, In this consists principally 
the difference between his glasses and those made in Eng- 
and. 
The difficulty hitherto 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 indistincily marked, that in large spectra the an- 
es could not be measured with a greater accuracy than 
from ten to fifteen minutes. In order to avoid this inconven- 
rence, Fraunhofer suceeded, by a very ingenious contrivance, 
_ in obtaining homogeneous light of each color in the spec- 
= 
