L. Bell — Absolute Wane-length of Light. 275 



Grating II has 8600 spaces with almost exactly the same 

 length and breadth as I, is free from ghosts and false lines and 

 like I, is very smoothly ruled, though it is somewhat inferior 

 to I in the matter of regularity. The definition is excellent 

 and the spectra alike in focus on both sides of the normal. It 

 was ruled in Nov., 1884 at ll°-6 C. 



Gratings III and IY are on speculum metal. The plates are 

 five inches square and five-eighths of an inch thick, and were 

 worked plane with especial care. The ruled surface is of the 

 same size in each, four inches long by two inches length of 

 lines. 



Grating III was ruled in April, 1885, at a very nearly con- 

 stant temperature of 10° C. It contains 29,000 spaces, having 

 very nearly the same grating space as II. It is a phenomenal 

 grating both in its superb definition and extraordinary regular- 

 ity of ruling, and was selected from a large number because of 

 its very unusual perfection. The focus of the spectra on each 

 side of the normal is the same and the ruling is flawless. 



Grating IY was ruled on the new dividing engine just com- 

 pleted by Prof. Rowland, and was one of the first large ones 

 completed. While the new engine has even now not received 

 the finishing touches, it has turned out a few gratings of re- 

 markable excellence. One of these is IY, which was ruled in 

 Dec, 1887, at a constant temperature of 17° *2 C. It contains 

 40,000 spaces within the same dimensions as III, is equal to it 

 in definition, and but very little inferior in regularity of ruling. 

 It has very nearly the same focus on both sides of the normal, 

 and the ruling is wonderfully even and perfect. 



It should be noted that these four gratings are widely di- 

 verse, being ruled a,t different temperatures and under different 

 conditions. I and II were ruled to widely diverse grating- 

 spaces on different parts of the screw, III was on speculum 

 metal and with more than six times the ruled surface of I or 

 II, and finally IY was ruled to a new grating space on a new 

 dividing engine. These differences may not favor close agree- 

 ment in the experimental results, but they certainly serve to 

 eliminate anything like systematic errors due to the gratings. 



The above gives a general view of the gratings employed, 

 but some further details will be mentioned in the second part 

 of this paper in connection with the determination of the grat- 

 ing spaces. 



On the Standards of Length. 



Yery many of the discrepancies in the determinations of ab- 

 solute wave-lengths are the direct result of uncertainty in the 

 standard of length employed. The cases of Angstrom and 

 van der Willigen have been already alluded to, and the same 



