494 M. A. J. Angstrom on a new Determination 



of Fraunhofer* s grating by 



0*00061 of a Par. inch, 



or by diminishing the number of lines by 5. 



That the second decimal is wrong in the above breadth 

 ( = 9*0155 lines) of Nobert's grating is not probable; far more 

 so is the supposition of an error ot about half this magnitude 

 in the estimation of the breadth of Fraunhofer's grating, espe- 

 cially since the microscope, forty years ago, had not reached its 

 present high degree of perfection. Fraunhofer, moreover, was 

 compelled to strengthen the extreme lines of his grating, in order 

 to see them more distinctly when measuring, a circumstance 

 which may possibly have affected the positions of these two lines. 



Besides the fact that my measurements agree with the results 

 which Fraunhofer obtained by -means of the grating No. 4, there 

 is another reason in favour of the assumption that the differ- 

 ences (a-7) arise from an incorrect value of e in Fraunhofer's 

 glass grating. For the above-cited memoir of Fraunhofer's 

 contains measurements made with another glass grating for 

 which e had the considerably greater value of 



0-0005919 of a Paris inch. 



Fraunhofer made no use of these measurements, probably 

 because this grating proved to be far less perfect, the spectra on 

 one side of the axis being twice as intense as those on the other. 

 Gn calculating these measurements, however, we obtain the fol- 

 lowing values corresponding to the lines from D to G : — 



D. 



E. 



F. 



G. 



Spectrum. 



2177-25 



2177-48 

 2177-64 

 2176-80 

 2177-55 



1947-21 

 1947-18 

 1947-23 

 1946-63 

 1947-25 



1796-10 

 1796-09 

 1795-99 

 1796-39 



1590-90 

 1591-07 

 1590-16 



5 

 4 

 3 

 2 

 1 



2177-34 



1947-10 



1796-14 



1590-71 







These values, compared with the series (a), indicate a constant 

 difference; here, however, the differences amount only to 



1-25, 1-14, 113, 1-63, 



or to about one-third of those last given. 



Now, since this last grating was nearly five times as coarse as 

 the former, and probably also broader, it must have been easier 

 to determine accurately its corresponding e. This circumstance 



