ZVIa^netic Separntious of Ii\)ii Lines in Different Fields. 5 



miglit liave (iisturbed tlio uniformity of the magnetic field in the 

 s]-)ark. An electromagnet witli water-coohng arrangement and 

 ft-rro-eohalt pole pieces would have been more suitable for 

 experiments in stronger fields. 



Echelon Grating. 



The echelon grating by Hilger, frequently used by Prof. 

 Nagaoka" in tJie study of tlie structure of mercury lines and 

 other works, was used m the present researcli also. Tiie 

 con-tant-; of tliis instrument are as follows : — 



Thickness of plate 9"350 mm., 



Number of plates 35, 



Steps I'O mm.^ 



Length 32-73 cm., 



A=T5ô5055, 



B = 5-9595x]0% 



C = l-95l4xlOi-, 



where. A, B, C are the constants in Cauchy's formula fen- the index 

 of refraction 



<« = -^ + ^ir- + -^7-' 



o 

 / being expressed in A.U. 



The wave number intervals corresponding to the distance of 



two ^uccessi^'e orders were calculated from the usual formula 



okmar 1 



o 



for the wave lengths of constant intervals of 50 A.U., and their 

 values are 2;iven in Table I. 



1) Nac^aoka and Takamine, Memoir of the Iaip3rial Acxiemy, S->c. II, Vol. I., Xo. 1, 1913. 



