L.Bell — Absolute Wave-length of Light. 361 



Grating I. 

 Sections 1 2 3 4 5 6 



Residuals, O^S 0'98 0-81 1-03 0-86 1'24 



Grating II 



Sections 1 2 3 4 5 6 



Residuals, 2^07 1'93 1'52 1-68 1-81 0*45 



Grating III. 

 Sections 123456789 10 



Residuals, 2^-80 2-85 1-77 2-77 2-70 2-77 2-67 2-64 2-73 2*77 



Grating IV. 



Sections 12 3456789 10 



Residuals, 0^-31 0-28 0-35 0'43 0'40 0-43 0'31 0-35 0'28 0'82 



The calibration of III is worth describing in detail. Cen- 

 timeter 3 was evidently too long. I therefore measured the 

 centimeters from 15 to 25 mm and from 25 to 35 mrn . The former 

 was quite normal but the latter showed. an increase almost iden- 

 tical with that of the whole third centimeter. I then examined 

 the grating in a strong light and detected at 27 mm from the end, 

 a faint line, such as usually indicates a few wavering lines caused 

 perhaps by dust under the diamond point. Placing, however, 

 this line under the microscope a band of perhaps twenty lines 

 appeared with spacing noticeably wider than usual. Here was 

 a very serious flaw in a grating to all appearance absolutely per- 

 fect. A most critical examination in the spectrometer of 

 course failed to detect it, but it was both detected and located 

 with unerring certainty by the process of calibration. Micro- 

 metrical measurements on this group showed an excess of about 



2 i " , 5 over an equal number of spaces elsewhere on the grating. 

 This quantity of course had to be taken account of in connec- 

 tion with the previous calibration. 



The deduction of the necessary corrections from the data 

 .given by calibration requires no little care and judgment, and 

 can be properly done only in connection with a detailed study 

 of the spectra given by various portions of the gratings con- 

 cerned. For the four gratings used by the author, these cor- 

 rections, applied directly to the lengths of the gratings in the 

 form of the quantity A before mentioned, are very nearly as 

 follows : 



Grating. A 



I ._ -0^-10 



II -+- 0'40 



III — 2-00 



IV + 0-45 



