882 
PEOFESSOE W. A. MILLEE ON THE PHOTOGEAPHIC 
lines, the second group three, and the third consists of eight or ten separate lines. This 
last is the most intense group that I have met with in the course of these experiments. 
Beyond this the spectrum is prolonged by a faint tail, which is strongest along the edges, 
and nearly vanishes midway between them ; this tail disappears a little beyond 150. 
(Fig. 28.) 
62. Aluminum . — This spectrum exhibits a characteristic group of dots at 106, another 
at 115'5, followed by others at irregular intervals as far as 155, beyond which no im- 
pression is visible. I have frequently repeated this experiment, as Professor Stokes 
informs me he that sees in the spectrum of aluminum lines which are beyond the limit 
of the zinc-spectrum, when the aluminum-spectrum is received upon a screen of uranium 
glass, or of a particular phosphate of uranium ; but either the collodion plate is 
insensitive to rays of this extreme refrangibility, or, as I believe, the quartz prism 
wliich I employ begins to fail in transparency at about the end of the zinc-spectrum. 
(Fig. 29.) 
53. Sodium . — The lines exhibited when sodium is employed appear to be mainly 
atmospheric; the usual nitrogen line at 110’5 is, however, developed into a broad band 
or group of nine or ten distinct lines, and the other atmospheric lines are very well 
marked ; the spectrum is prolonged into a faint nearly continuous band at about 156, 
when it ceases to be visible. (Plate XL. fig. 47.) 
54. Potassium . — The spectrum produced by potassium presents the same appearances 
as that of sodium, both being apparently only atmospheric (fig. 46). 
[55. lAtliium . — The spectrum of this metal differs from those of the other alkali- 
metals in presenting a single well-marked group of dots at about 123 ; it is prolonged into 
a tail, which resembles that of magnesium, and fades out at about 150. (Fig. 48.) — 
Feb. 1863.] 
56. It is unnecessary to give any details of experiments made with electrodes one of 
which consisted of one metal and the other of a different metal. Under these cu’cum- 
stances the lines starting from the side corresponding to each metal are identical with 
those furnished by the particular metal. This mode of making the experiment is 
therefore frequently convenient when it is desirable to compare the spectrum of any 
given metal with another selected for comparison. When the difference in volatility 
between the two is extreme, as when platinum is opposed to mercury (fig. 10), it may 
happen that one spectrum only is seen, the lines starting from an edge of the photogra- 
phic impression, and terminating at irregular distances before they reach the opposite 
edge. 
b. Spectra of Alloys. 
57. The principal object of these experiments was to determine the influence which 
small amounts of foreign metals exercise upon the photographic image. When equal 
weights of the two metals are employed (tin and lead, for example, or cadmium and 
lead), a compound spectrum exhibiting the lines due to both metals is produced ; and it 
is not always the more volatile metal that predominates. An alloy containing 62 parts 
