182 H, A. Rowland—Relative Wave-length. 
probable error very materially and therefore defers, for the 
present, further discussion of the problem. 
Through the courtesy of Mr. Peirce the writer has been 
enabled to test the legitimacy of the above correction and at the 
same time check his own results. Mr. Peirce kindly forwarded 
his gratings and standard of length for examination and com- 
parison, and the results were decidedly instructive. 
Grating ‘“‘H,” with which a large part of the work was done, 
showed, as was suspected, a local error, equivalent to a correc- 
tion of one part in 55000 in the resulting wave length. Tested 
in the spectrometer, the portion including the grror showed a 
grating space distinctly greater than that of the grating taken 
as a whole, showing thus both the necessity and the algebraic 
sign of the correction. The other gratings showed similar 
errors varying in amount, but the same in sign, the correction 
requiring in every case a reduction in the wave length. The 
abnormal portion was invariably at one end or the other of the 
grating concerned, never in the middle. 
The standard of length used by Mr. Peiree—‘ No. 3” a glass 
decimeter—was compared with S{ and S§ and the preliminary 
results show that the length assigned to it was too great by: 
very nearly 24, 1 part in 50000. Now the wave length of D, 
as deduced from grating H was, 
5896°26 
Less error of ruling - - —'10 
Less error of “No. 3” —12 
Corrected value- ---- 589604 in air at 30 in. pressure and 70° F; 
which shows a tolerably close correspondence with the results 
obtained by the writer. A more complete discussion of Peirce’s 
results is reserved until the relation between ‘No. 3” and S84 
and S% shall be more exactly known. The latter standards 
would appear to be the more trustworthy, since they are based 
on various independent determinations, while ‘‘ No. 3” is based 
on an indirect comparison with meter “No. 49,” a standard 
concerning the exact length of which there seems to be some 
little doubt. 
ART. XXI.—On the Relative Wave-length of the Lines of the 
Solar Spectrum; by Henry A. ROWLAND. 
For several years past I have been engaged in making a pho- 
tographic map of the solar spectrum to replace the ordinary 
engraved maps and I have now finished the map from the 
extreme ultra violet, wave length 3200, down to wave length 
5790. In order to place the scale correctly on this map, I have 
