Wave-length of Light. 281 



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 ruling should appear in (8001) and (80014). A similar 



o 



study of the gratings used by Angstrom would be of no little 

 interest. 



The other determination alluded to is one by M. de 

 Lepinay, 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 litre to the decimetre, 

 a ratio not at present exactly determined. 



The results detailed in this paper are in a certain sense 

 preliminary. The writer hopes that in the near future, ex- 

 periments with metallic gratings will enable him to lessen the 

 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 comparison, 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 

 correction of one part in 55,000 in the resulting wave-length. 

 Tested in the spectrometer, the portion including the error 

 showed a grating-space distinctly greater than that of the 

 grating taken as a whole, showing thus both the necessity for 

 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. Peirce (" No. 3 " a 

 glass decimetre) was compared with Sj and S^; and the pre- 

 liminary results show that the length assigned to it was too 

 great by very nearly 2/x, 1 part in 50,000. Now the wave- 

 length of D 1; as deduced from grating H, was 



5896-26 

 Less error of ruling . . —'10 

 Less error of " No. 3 " . - -12 



Corrected value .... 5896*04 in air at 30 in. pressure 



and 70° F.; 



