L Bell — Absolute Wave-length of Light. 269 



consistent. Bnt in spite of all Angstrom's care the event has 

 shown that his wave lengths are in error by as much as one 

 part in seven or eight thousand mainly through an error in the 

 assumed values of his standards of' length. Angstrom measured 

 his gratings by means of a dividing engine the screw of which 

 was very exactly determined by comparisons resting on the 

 Upsala meter which, in turn, had been compared by M. Tresca 

 with the prototype of the Conservatoire des Arts et Metiers. 

 Had this comparison given the correct value _of the Upsala 

 meter Angstrom's wave-lengths would have been very nearly 

 exact except for corrections due to errors of ruling in the grat- 

 ings. 



After Angstrom's research the question of absolute wave- 

 length was not seriously raised for ten years, when Mr. C. S. 

 Peirce under the auspices of the United States Coast Survey 

 again attacked the problem, armed with Rutherfurd gratings 

 far superior to those used in any previous research. No official 

 report of his very elaborate and exhaustive experiments has 

 ever been published save a very brief preliminary report in the 

 American Journal of Science in 1879. Such of his results as 

 have been made in any way public will be discussed in the 

 experimental part of the present paper. 



Meanwhile Thalen, who so efficiently aided Angstrom in his 

 work, has taken up the part of it left uncompleted by the lat- 

 ter' s death and in his paper " Sur le Spectre du Fer," published 

 at Upsala in 1885, has discussed the corrections which must be 

 applied to Angstrom's values by reason of the error in the 

 Upsala meter. It seems that through the experiments of Pro- 

 fessor Lindhagen, Angstrom became aware as early as 1872 

 that the assumed value of his standard was considerably too 

 small. His death prevented his verification of M. Lindhagen's 

 results and nothing further was done till Thalen took up the 

 work. Tresca' s comparisons had shown that the true length of 

 the Upsala metre at 0° was 999 , 81 mm . But the very exact ex- 

 periments of M. Lindhagen have shown the above to be some- 

 what too small and that the correct value is 999 "94. This dif- 

 ference makes, of course, a marked error in the wave-lengths 

 based on Tresca's results. Applying the appropriate correc- 

 tion^ the wave length of E, the line most carefully determined 

 by Angstrom, becomes 



5269-80, 

 instead of 'the original 5269 '12. 



This final result of Angstrom is certainly entitled to con- 

 siderable respect and seems to be subject only to those correc- 

 tions which may be due to irregularities in the gratings. These 

 were, however, so poor compared with the gratings of to-day, 



