328 
PROFESSOR W. N. HARTLEY ON SPECTRUM PHOTOGRAPHY 
Of the quintuple group the first and fifth lines, wave-lengths 2781 *8 and 2775*5, 
are shortened by two-thirds, the third line, wave-length 2778'7, is shortened by 
one-lialf. The third is just barely visible as a continuous line when one knows where 
to look for the weakened portion. 
(3.) 0'03 per cent. Mg,, or 30 parts per 100,000 of solution. 
The least refrangible line, wave-length 4480, has disappeared. The first and third 
lines of the triplet, wave-lengths 3837*9 and 3831'2, are still visible but very weak. 
The second triplet has disappeared. 
The doublet, 2935*9-2928*2, is barely a continuous pair of lines, the centres being 
very much enfeebled. The line 2851'2 is not very much altered, being of equal 
strength throughout its length instead of having a thickened appearance at one pole. 
The quadruple group have not undergone any great change, but the nimbus or 
halo is extinguished as in the preceding case. Of the quintuple group there is a very 
faint indication only of the lines with wave-lengths 2781'8 and 2778*7, the others 
are extinct. 
(4.) 0*02 per cent. Mg., or 20 parts per 100,000 of solution. 
All the lines are weakened. 
The line 2781*8 is extinct. 
(5.) 0*01 per cent. Mg., or 10 parts per 100,000 of solution. 
The lines are still further weakened, and those with wave-lengths 3837*9 and 3832*1 
have become extinct. 
The doublet, wave-lengths 2935*9 and 2982*2, is shortened by two-thirds the 
length of the lines. 
The line 2851*2 is attenuated. 
Of the quadruple group the second and fourth, wave-lengths 2796*9, 2789*6, are so 
attenuated as to be barely continuous. Scarcely a trace of the quintuple group is 
visible. 
(6.) 0*001 per cent. Mg., or 1 part per 100,000 of solution. 
Of the doublet, the first line is shortened by two-thirds, wave-length 2935*9, the 
second is weakened and shortened by nearly three-fourths of its length, wave-length 
2928*2. 
The line 2851*2 is weakened throughout and more especially through one-half its 
length, it is now barely a continuous line. 
Of the quadruple group the first and third lines, wave-lengths 2801*6 and 2794*1, 
are attenuated through one-half their length to a great degree and are weakened 
throughout. The second and fourth lines, wave-lengths 2796*9 and 2789*6, are 
shortened by one-half their length. The difference between this spectrum and that of 
the solution ten times as strong is shown to be the result of strictly gradational 
