Pottox anp Lronarp—On the Spectra of Cerium, &c. 269 
DipymMiumM. 
A photograph was taken with the solution rich in didymium, 
but still containing some lanthanum, and giving a strong spectrum 
of the latter element. The slit was then lengthened, and a 
photograph of the spectrum of pure lanthanum superimposed on 
the first. The long lines of the second spectrum thus cancelled 
all the lanthanum lines in the first spectrum, leaving the didymium 
lines as short lines between. 
The following are the measurements of the most characteristic 
lines, and those marked with an asterisk appeared to be the most 
persistent :— 
Some Prrsisrent Lines or Dipymium CHLORIDE. 
Linear Intensity 
Measurement, Wavye-length. and 
zo inch. Persistency. 
279°9 4792°8 Au No. 9 
293-7 | 4542°8 5 
298-1 | 4463°2 9 
308°8 | 4303°8 10” 
317:9 | 4177-5 10” 
399°9 3383°0 Au No. 15 
475°1 2946°8 3* 
485°3 2599°9 one 
490°6 287671 3 
502°4 2825-6 Au No. 20 
CuemicaL Lagoratory, 
Royat CoLLeGEe or ScrEeNcE, 
Dupin. 
