878 
PEOEESSOE W. A. MILLEE ON THE PHOTOGEAPHIC 
As might be anticipated, the spectra of the more volatile metals are the most intense — 
those of bismuth and antimony, of cadmium, zinc, and magnesium being especially 
remarkable in this respect. 
A certain similarity is also observable in the spectra of allied metals, as in the case of 
the three metals last mentioned, also in those of iron, cobalt, and nickel, of bismuth 
and antimony, as well as of chromium and manganese. 
It should further be observed that, in estimating the apparent length of the different 
spectra, considerable difficulty is frequently experienced owing to the extremely faint 
impressions Avhich the most refrangible rays commonly occasion ; in some experiments 
this portion of the spectrum with the same metal appears to be longer than in others 
made under apparently similar conditions. 
28. Platinum. — The characters of the spectrum of platinum are feebly marked. The 
atmospheric lines are well developed. It is important to remark that the continuous spec- 
ti'um which forms the background to all the metallic spectra in their less refrangible por- 
tion, appears to be due to the oxygen and nitrogen of the atmosphere. Between 96 ‘5 and 
I03'6 this spectrum is intense, and is crossed by a strong pair of compound lines followed 
by three strong groups of lines, ending in two other groups, each successively of less in- 
tensity than the preceding ones. At 10 7' 5 is a well-defined line, and a strong band at 
llO'D. Three others, nearly equidistant, follow at 114, 117‘3, and 121*7 : at 137 and 
138*2 are two faint lines, and beyond these, at 142*5, is a very faint pau* of lines. All 
these are atmospheric lines. The true metallic spectrum of platinum (Plate XXXD^ . 
fig* 5) gradually fades out with a series of groups of dots, and terminates at about 162. 
29. Iridium. — The spectrum of this metal, for a fine sample of which I am indebted 
to my friend Professor Wheatstone, scarcely differs sensibly from that of platinum. 
The lines apparently most prominent are atmospheric. (See Plate XL. fig. 44.) 
30. Palladium. — This spectrum is much more uniform in intensity than that of 
platinum. Besides the usual atmospheric lines, groups of dots commence at 103*5, and 
continue at irregular intervals. Four of the strongest of these groups occur at 138*5, 
143, 148, and 152*5, measuring at about the centre of each group. The spectrum ter- 
minates rather abruptly at about 162*5. (Plate XXXIX. fig. 6, & Plate XL. fig. 38.) 
31. Gold. — The spectrum of this metal has about the same degree of intensity as that 
of palladium, and it is almost exactly equal to it in length. Amongst its numerous 
lines, the most characteristic are two pahs of dots very close together at 124, and three 
groups of dots, commencing at 130*0, 143*5, and 153*5 respectively; the last is faint. 
(Plate XXXIX. fig. 7.) 
32. Silver. — The spectrum of this metal is very characteristic. It is intense for the 
first third of its length, then becomes much fainter, after which several remarkable and 
very strong groups of lines are observed, beyond which the impression disappears at 
170*5. The most conspicuous lines are a pair of double lines interrupted in the middle 
groups of dots rather strong, at 126*8, 129*0, and 131*0; between 140*5 and 144*5 are 
three very strong groups of dots. About 148 is another broad strong group; beyond 
