348 



L. Bell — Absolute Wave-length of Li glit. 



Date. 



Number of series. 





Angle. 



Nov. 3, 



1 



4o 



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 mm , so that no correction was required 

 for this cause. Weighting and combining the above observa- 

 tions the final value is 



^ = 45° 1' 48 ,y -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 tempera- 

 ture of very nearly 20° and an average pressure of 760 mm . 

 Thirty-six series of observations were obtained in the fourth 

 order, as follows : 



Date. 



Number of series. 



Angle. 



Mar. 6, 



2 



42° 5' l"-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 



Combining and weighting, the mean value is : 



cp = 42° 4' 59"-28db0"-2. 



The probable error is equivalent to about one part in six hun- 

 dred thousand in the wave-length. 



Both the glass gratings were used exclusively for the line 

 Dj, which was on the whole most convenient for measurement, 

 D 2 being rejected by reason of the troublesome atmospheric 

 lines. The relative wave-lengths of a very large number of 

 lines have been so exactly determined by Prof. Rowland that 

 any one of them would have given results equally valuable, and 

 in the subsequent work with gratings III and IV, two of these 

 standard lines were employed. 



In this second part of the investigation, the gratings as before 



