IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
257 
all others that I have yet observed) the lines as in the before-mentioned case of the 
triple line (b) of magnesium, are of unequal length, and that in the new method of 
Yiolet. 
Fig. 3. 
Red. 
Lead. 
Cadmium. 
observation the lines in the spectra of the two metallic vapours and of the air are 
separated in the clearest and most convenient manner, the air lines going right across, 
and the lines of the metallic vapours extending to greater or less distances from each 
pole, and in some cases (i. e. of the longest lines) overlapping. 
With this communication are maps (Plates XI., XII., XIII.) of the spectra of the 
following elements made on this method, the jar being used : — Na, Li, Mg, Al, Mn, Co^ 
Ni, Zn, Sr, Cd, Sn, Sb, Ba, and Pb. The lines were laid down from Thalen’s maps, 
given in the memoir quoted above and on the same scale, namely 2 centimetres to each 
TooVoo Hiillim. of wave-length. The spectra were then carefully and repeatedly observed, 
and the comparative lengths of the lines estimated and laid down over their respective 
wave-lengths 
At the same time that these spectra have been mapped with the spark taken in air, 
many of them have also been observed when their metals were enclosed in tubes and 
subjected to a continually decreasing pressure, as in the case of the before-mentioned 
experiment with magnesium. In all these experiments it was found that the longest 
lines invariably remained visible longest. 
In the case of zinc the effect of these circumstances was very marked, and they may 
be given as a sample of the phenomena generally observed. When the pressure-gauge 
connected with the Sprengel pump stood at from 35 to 40 millimetres, the spectrum at 
the part observed was normal, except that the two lines 4924 and 4911 * (both of which, 
when the spectrum is observed under the normal pressure, are lines with thick wings) were 
considerably reduced in width. On the pump being started these lines rapidly 
decreased in length, as did the line at 4679, — 4810 and 4721 being almost unaffected ; 
at last the two at 4924 and 4911 vanished, as did 4679, and appeared only at intervals 
as spots on the poles, the two 4810 and 4721 remaining little changed in length though 
much in brilliancy. This experiment was repeated four times, and on each occasion the 
gauge was found to be almost at the same point, viz. : — 
* Thalen’s scale as given by Watts. 
