368 Mr. Louis Bell on the Absolute 



my own. The cause of this discrepancy is partly due to the 

 varying temperature under which the glass gratings were 

 first measured, and partly to the change in the value assigned 

 to the standard of length.* 



Then, too, the corrections applied to gratings II. and III. 

 may be slightly in error. Taking into account all these 

 sources of uncertainty, it is my opinion that the above final 

 result is not likely to be in error by an amount as great as one 

 part in two hundred thousand. 



Taking the above value of the absolute wave-length and 

 applying the appropriate corrections to some of the funda- 

 mental lines given in Prof. Rowland's paper (this Journal, 

 March 1887), the wave-lengths of the principal Fraunhofer 

 lines in air at 20° and 760 millim. are, 



. ( line between " head " and") 7621-31 

 \ " tail " of group J 



B „ 6884-11 



C 6563-07 



B l ' . 5896-18 



D 2 5890-22 



E x 5270-52 



E 2 5269-84 



b, 5183-82 



F 4861-51 



Comparisons between these wave-lengths and the older 

 ones become somewhat uncertain toward the ends of the 

 spectrum, since the appearance of lines like A, B, Gr, and H 

 varies greatly with the dispersion employed. The relative 

 wave-lengths from which the above values are calculated are 

 certainly exact to within one part in half a million. 



It may not be out of place here to discuss the most 

 recent work on this problem. Just before the publication of 

 my first paper the very elaborate paper of Miiller and Kempf 

 appeared. Their work is a monument of laborious research, 

 and it is unfortunate that so much time should have 

 been spent in experiments conducted with glass gratings of 

 small size and inferior quality. Since the invention of the 

 concave grating it is a waste of energy to make micrometric 

 measurements with plane ones, and this statement could 

 hardly be corroborated more strongly than by the relative 



* In terms of the length I originally assigned to S a . 2) the wave-length of 

 D t would he 5890-14, while if the value deduced from the Berlin com- 

 parison were taken it would be 5896-22. The wave-length quite certainly 

 lies between these values, but the proper weight to be given to the Berlin 

 comparison relatively to the others is rather uncertain. 



