Wave-length of Liglit. 351 



Grating I. was used during October and November 1886, 

 and forty-eight series of observations were obtained as follows, 

 each series consisting of three to seven observations: — 



Date. 



Number of 



An°le. 





series. 







Oct. 19. 



1 



o 



45 



1 47-2 



20. 



1 



45 



1 48-4 



22. 



2 



45 



1 48-2 



23. 



1 



45 



1 49-8 



26. 



4 



45 



1 49-3 



27. 



3 



45 



1 48-2 



31. 



1 



45 



1 50-1 



STov. 3. 



1 



45 



1 48-6 



4. 



3 



45 



1 47-4 



5. 



2 



45 



1 47-9 



10. 



4 



45 



1 47-8 



11. 



6 



45 



1 49-7 



16. 



8 



45 



1 48-2 



17. 



5 



45 



1 47-5 



20. 



6 



45 



1 47-5 



Grating I. was used at an average temperature of very 

 nearly 20°, to which all observations were reduced. The 

 average barometric height was 761 millim., so that no cor- 

 rection was required for this cause. Weighting and com- 

 bining the above observations, the final value is 



0=45° V 48"-24±0"-ll, 



corresponding to the spectrum of the third order. 



The resulting probable error in wave-length is about one 

 part in a million. 



Grating II. was used in March 1887, at an average tem- 

 perature of very nearly 20°, and an average pressure of 

 760 millim. Thirty-six series of observations were obtained 

 in the fourth order, as follows: — 



Date. 



Number of 





Angle. 





series. 







Mar. 6. 



2 



42 



5 1-2 



10. 



1 



42 



4 58-6 



11. 



7 



42 



5 1-4 



15. 



1 



42 



5 4-0 



16. 



6 



42 



4 57-8 



17. 



6 



42 



4 58-5 



18. 



7 



42 



4 59-1 



23. 



6 



42 



4 58-3 



