THE ULTRA-VIOLET SPECTRA OP THE ELEMENTS. 
215 
Potassium lines. 
Approximate 
wave-length. 
Remarks. 
3445-0 1 
3443-6 / 
3216-5 
3101-0 
3033-0 
2992-0 
2963-4 
2942-0 
Double line. 
All are easily reversed. 
The lines become weaker as they are more refrangible. 
a 
Sodium. 
The sodium lines observed by us also form one apparently harmonic series with the 
double line, wave-length 3301, observed by Cornu. In this case also we have not 
been able to make out that any of the lines above 3301 are double, as when there is 
enough sodium present to develop them decidedly they are always more or less 
diffuse and reversed. Indeed, the line at 3301 is a very close pair and it is not often 
seen as two lines. 
One line is so near to the very strong magnesium line, wave-length 2852, that the 
apparent development of the magnesium line by sodium was for some time an enigma 
to us. The sodium line is a little less refrangible than the magnesium line. 
Sodium lines. 
Approximate 
wave-length. 
Remarks. 
3301-0 
2853-3 
2679-0 
2593-3 
Cornu’s double line. 
All the lines are easily reversed. 
Lithium. 
We have already described one apparently harmonic series of lithium lines extend¬ 
ing into the ultra-violet up to about wave-length 3799. This series we described as 
all single lines though alternately sharp and diffuse. This description is correct, 
except that we have since found that one line of the series, namely, the strong blue 
line at wave-length 4604, is really a double line. When a fresh dose of lithium, or of 
some one of its compounds, is introduced into the arc, a second weaker line comes out 
