L. Bell— Absolute Wave-length of Light. 181 
on account of its very small error of ruling, we have finally for 
the wave length of D, at 20° C. and 760™™ pressure, 
5896:08 or in vacuo, 5897°71 
It is no easy matter to give any well founded estimate of the 
probable error of the above result. So far as experimental 
errors are concerned the result with either grating should be 
correct to one part in two hundred and fifty thousand, but 
the error in the gratings introduces a complication by no means 
easy to estimate. As nearly as the writer can judge, however, 
it seems probable that the error of the final result does not 
exceed one part in two hundred thousand. For comparison, 
the values deduced from the work of Peirce and of Angstrém 
are subjoined. 
Micrometer measure by Rowland, from Peirce’s prelimi- 
TTA OEE SUG Re Sew ee ce Fe RAINY Np ee ML NS UE 5896°22 
Thalen’s correction of Angstrém- SUID BeNOR DUIS ees ae ay 28 5895°89 
both being in air at ordinary temperature and 760™, 
As neither result was corrected for errors in the gratings the 
cause of the discrepancy is obvious. 
T'wo determinations of absolute wave length have been pub- 
lished since this work was undertaken by the writer. One is a 
very elaborate one by Miller and Kempf, who employed four 
gratings by Wanschaff and used the method of minimum devia- 
tion. Their results were as follows: 
Gratings 222). (2:05 1) (5001) (8001) (80014) 
Wave length... 5896°46 5896°14 5895°97 5896°33 
By a correction founded on the unwarrantable assumption 
that the mean value was correct the above results are brought 
into apparent agreement. Nothing, however, short of a study 
in detail of each grating can furnish data for obtaining any- 
thing like an accurate result from the above figures. It would 
seem that (5001), which had the smallest probable error, should 
show but a trifling error of ruling, while one would expect to 
find a portion or portions of (2151), in which the grating space 
is abnormally large. Corresponding errors of raling ‘should 
appear in (8001) and (80014). A similar study of the gratings 
used by Angstrém would be of no little interest. 
The other determination alluded to is one by M. de Lépinay, 
using a quartz plate and Talbot’s bands. Without discussing 
the method it is sufficient to say that the result obtained 
depends on the relation of the liter to the decimeter, a ratio not 
at present exactly determined. 
The results detailed in this paper are in a certain sense pre- 
liminary. The writer hopes that in the near future, experi- 
ments with metallic gratings wili enable him to lessen the 
