364 L. Bell — Absolute Wave-length of Light. 



Taking the above value of the absolute wave-length and ap- 

 plying the appropriate corrections to some of the fundamental 

 lines given in Prof. Rowland's paper (this Journal, March, 1886) 

 the wave-lengths of the principal Fraunhofer lines in air at 20° 

 and 760 mm are, 



A/ line between "head"\ . *7ft01 »Q1 



Und "tall " of group/ / 0<S 1 Ol 



B " 6884-11 



C . 6563-07 



D t ... 5896-18 



D„ ^ 5890-22 



E x " 5270-52 ' 



E„ 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, G and H vary so much 

 with the dispersion employed. The relative wave-lengths above 

 given 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 con- 

 ducted with glass gratings of small size and inferior quality. 

 Since the invention of the concave grating, it is a waste of en- 

 ergy to make micrometric measurements with plane ones, and 

 this statement could hardly be corroborated more strongly than 

 by the relative wave-lengths given by Miiller and Kempf. The. 

 probable error of their wave-lengths is in general not less than 

 one part in two hundred thousand. That the value assigned 

 by them to the absolute wave-length is as near the truth as it 

 probabty is, is due to no lack of faults in the gratings. Their 

 results for the line D 1 were as follows : 



Grating. W. L. 



"2151 " 5896-46 



"5001" 5896-14 



"8001 " 5895-97 



"8001L" 5896-33 



A discussion of these errors as exemplified in the paper 

 under consideration would take up too much space to be in- 

 serted here, but one or two points are worthy of notice. 

 When a grating gives different results in the different orders, 

 it is evident that there are in it serious errors of ruling, and 

 the maximum amount of the variation will give a rough esti- 



