Wave-length of Light. 211 



the rulings. It was therefore necessary to know very exactly 

 the angle between this line and the direction of the individual 

 rulings ; in other words, the angle between the line of motion 

 of the grating and the direction of the diamond stroke in the 

 dividing-engine. This was ascertained by means of two test 

 plates each some twelve centim. long ruled in centims., and 

 then superimposed line for line. By measuring the minute 

 distances between each end of a pair of superimposed lines, 

 the length of the lines and the amount by which their ends 

 overlapped at each end of the test plate, the required angle 

 could be deduced with great exactness. It differed so little 

 from 90°, however, that the correction produced, barely one 

 part in a million, was entirely negligible. 



After all reductions and corrections, the following series 

 of values were obtained for the grating- spaces of gratings I. 

 and II. : — 



Series. 



Grating 1. Standard, 

 millim. 



1. 

 2. 



3. 

 4. 

 5. 



0-00250023 SJ 

 0-00250016 „ 

 0-00250013 „ 

 0-00250015 „ 

 0-00250018 „ 



6. 



7. 

 8. 

 9. 



0-00250021 S? 

 0-00250023 „ 

 0-00250023 „ 

 0-00250023 „ 



Mean value adopted after weighting and combining the 



above observations was 



0-002500194 millim. + 10. 

 The probable error thus appears to be not far from one part 

 in two hundred and fifty thousand. The difference in the 

 results obtained from the two standards seems to be purely 

 accidental, as appears from the measurements on grating II. 



jries. 



Grating II. 



Stand 





millim. 





1. 



0-00351888 



« 



2. 



0-00351883 



?j 



3. 



0-00351885 



>j 



4. 



0-00351886 



?j 



5. 



0-00351883 



» 



6. 



0-00351893 



$2 



7. 



0-00351888 



5j 



8. 



0-00351888 



jj 



9. 



0-00351888 



»» 



Mean adopted, 0-003518870 + 10. 



