the Occurrence of Enhanced Lines in the Arc. 859 



b. Uviol spectrograph. A3170-X5000. 



Sources of light : ordinary direct arc and water arc. 



\\ 3250 Well marked in water arc . absent from ordinary arc; 



characteristic spark line. 

 3526 Strong in ordinary arc ; much weakened in water arc: 



impurity ? 

 3536 Well marked in water arc ; absent from ordinary arc ; 



characteristic spark line. 

 3650 Fainter in water than in ordinary arc. "") 

 3729 Slightly stronger in water arc. 

 3982 Well marked in ordinary arc ; barely [ Characteristic 



visible in water arc. f arc lines, 



4307 Very well marked in ordinary arc ; [ 



much weakened in water arc. J 



4416 Strong in water arc ; absent from ordinary arc ; enhanced 



line. 



The following additional lines undergo no change on 

 passing from the ordinary arc to the water arc : XX 3253, 

 3261, 3404, 3466, 3468, 3611, 3613, 3614, 4413, 4662, 

 4678, 4800, and 5086. 



Glass spectrograph. AA 3600-7000. 



1 



Ware- 

 lengths 



Ordinary arc 



between 



Carbon cathode 



Water arc 



between 



Graphite anode 



Remarks. 



in 

 I. A. 



and 

 Cd anode. 



and 

 Cd cathode. 





4306-8 



8 en 



3 en, br in I 





4413-0 



8 cs 



4 cs 





44157 



2 cs 



5 ds 



enh. 



<\il 



Ocn 



00 en 





4662-3 



10 c 



5 cnbr 





4678-2 



20 c 



12 c, brv in I 





4799-9 



20 c 



15 c, brv in I 





5085-8 



20 c 



20 c, brv i n I 





51547 



8cs 



6c 





5337-9 



000 c 



2dd n 



enh. 



53789 



00 c 



3dd n 



enh. 



6325-2 



6 cs 



3c 





64385 



20 c 



20 c, b^ i n I 



I 



§ 9. Explanation of Plates. 



The reproductions have been made from enlargements of 

 the original photographs. Plate XVII. shows the variations 

 in the character of the spectrum of magnesium in the region 

 comprised by the two triplets X 3883 and X 5184 on passing 

 from the ordinary direct arc in gaseous air (a) to those under 



