6 Memoir of the Life of M. Frmnihofer. 



discovered the numerous causes which occasioned 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 labour. He 

 found also, that with whatever degree of accuracy he followed 

 the theory in the construction of achromatic object-glasses, 

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

 was convinced that it was wrong to neglect certain quantities, 

 such as the thickness of the lens and the higher powers of 

 the apertures, merely to obtain commodious formulae ; and on 

 the other hand, there was no exact method for determining 

 the exponents of refraction and dispersion in the glass, used 

 for achromatic object-glasses. The first of these inconve- 

 niences he avoided by a new method, in which he neglected 

 no quantity upon which the required degree of exactness de- 

 pended. 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 his glasses and those made in England. 



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 colour to another was so 

 gradual, and indistinctly marked, that in large spectra the 

 angles could not be measured with a greater accuracy than 

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

 nience, Fraunhofer succeeded, by a very ingenious contriv- 

 ance, in obtaining homogeneous light of each colour in the 

 spectrum. In these experiments, he discovered in the orange 

 compartment of the spectrum, produced by the light of the 

 fire, a bright line, which he afterwards found to exist in all 

 spectra, and by means of which he was enabled to determine 

 the refractive powers of the bodies which produced them. 



By using prisms entirely exempt from veins, — by carefully 

 excluding all extraneous light, and even stopping those rays 



