370 Mr. Louis Bell on the Absolute 



obtained from each grating, and so being able to apply the 

 corrections found. Peirce's method was such as should have 

 secured very excellent results, and such will undoubtedly 

 follow a further investigation of the standards and gratings. 

 Still another recent determination is that by Kurlbaum, who 

 used two good-sized speculum-metal gratings and measured 

 them with particular care. Like the previous experimenters, 

 he neglected, although he did not ignore, the errors of ruling, 

 and consequently the results he obtained are somewhat in 

 doubt. A serious objection, moreover, to his work is the 

 very small spectrometer he used. To undertake a determi- 

 nation of absolute wave-length with a spectrometer reading 

 by verniers to 10" only, and furnished with telescopes of 

 only one-inch aperture is simply courting constant errors. 

 More especially is this true, since it would be hard to devise 

 a method more effective in introducing the errors of ruling 

 than to use a grating with telescopes too small to utilize its 

 full aperture, and then determine the grating-space by 

 measuring the total length of the ruled surface, Kurlbaum's 

 gratings, too, were of an unfortunate size, 42 and 43 millini. 

 broad respectively, and consequently by no means easy to 

 measure. On the whole his result, 5895 ,l J0, is not surprising. 



The agreement of relative wave-lengths as determined by 

 different experimenters unfortunately gives no measure as to 

 the accuracy of the work. The relative wave-lengths, as 

 determined by Miiller and Kempf and by Kurlbaum, agree in 

 general to within 1 part in 100,000; the absolute wave-lengths 

 assigned by these experimenters vary by more than 1 part in 

 30,000. 



A very ingenious flank movement on the problem of ab- 

 solute wave-length has been made by Mace de Lepinay. His 

 plan was to use interference-fringes in getting the dimensions 

 of a block of quartz in terms of the wave-length, and then to 

 avoid the difficulties of the linear measurement by obtaining 

 the volume through a specific-gravity determination. His 

 results do not indicate, however, experimental accuracy as 

 great as can be obtained by the usual method, and the final 

 reduction unfortunately involves a quantity even more un- 

 certain than the average standard of length, i. e. the ratio 

 between the metre (?) and the litre. 



It may be interesting here to collect the various values 

 which have been given for the absolute wave-length within 

 recent years. Kesults are for the line D 1# 



