2QQ Prof. E.Wiedemann on the 



the ratio of the true coefficients of emission multiplied by 

 (A.-XO/A. 



We may determine i'/i" directly, if we observe with a 

 spectrophotometer possessing so great a dispersion that the 

 line also appears as a continuous band, for then the slit 

 appears illuminated by homogeneous light of a width which 

 is distinctly smaller than the spectral image of the line, it 

 behaves therefore as a portion of a continuous spectrum ; 

 instead of X 2 ~^i we have A, and that the more accurately 

 the greater the dispersion, and the ratio of brightness measured 

 in the apparatus is itself i'/i". 



In order to determine the ratio i'/i" for a sodium flame and 

 the amyl-acetate lamp, the following arrangement was made: — 

 The spectrum apparatus consisted of two telescopes belonging 

 to Herr Ebert and a plane Rowland's grating. The colli- 

 mator was provided with an arrangement for symmetrically 

 narrowing down the slit, and had an aperture of 65 millim. 

 and a focal length of 1 metre ; the observing-telescope an 

 aperture of 75 millim. and a focal length of 1 metre. The 

 grating had a divided surface of 46 x 36 millim. and possessed 

 very good definition. The slit was made very narrow. For 

 the determination of the maximum of brightness a totally- 

 reflecting prism was placed before one half of the slit ; the 

 beam of light from an arc lamp fell upon the uncovered half of 

 the slit, after having traversed two adjustable Nicols. The 

 light from the sodium flame or amyl-acetate lamp entered 

 the apparatus through the totally-reflecting prism. First the 

 sodium flame was brought before the prism, the apparatus 

 adjusted for the greatly expanded sodium lines of the fourth 

 spectrum by turning the grating, and the spectrum of the 

 arc lamp weakened by turning one of the Nicols until it 

 appeared of equal brightness with the sodium lines in that 

 part of the spectrum. 



Then the sodium flame was replaced by the amyl-acetate 

 lamp, and adjusted for the yellow of the first diffractive 

 spectrum. Since the light of the electric lamp was much 

 brighter than that of the amyl-acetate lamp it was weakened 

 by clouded glasses of known strength till the two spectra 

 were of equal brightness. A direct comparison of brightness 

 between the amyl-acetate lamp and the sodium flame in the 

 fourth spectrum was not possible, because in the fourth 

 spectrum the first was hardly visible. But since for all 

 sources of white light the degree of weakening in passing 

 from one spectrum to another is the same, and all continuous 

 spectra are dispersed in the same degree, the ratio of bright- 

 ness of amyl-acetate to arc-light must be the same in the 



