334 ApENEY—Photographs of Spark Spectra from the Rowland Spectrometer. 
ships of the component lines of spectra, but also to the modes of vibration of the 
matter, which is the cause of such radiations, the evidence of such molecular 
movements being the phenomena exhibited when the lines are under powerful 
- magnetic stress, it is clearly of importance that published reproductions of photo- 
graphed spectra should show any characteristics of the component rays as well 
as the constitution of the spectra generally. In confirmation of this statement it 
may be remarked that the late Dr. Preston, r.R.U.1., F.R.s., found the photographed 
spectra, which form the subject of this communication, and others belonging to 
the same collection, of great assistance in his important work upon the influence 
of a strong magnetic field upon radiating matter. It is desirable, therefore, that 
prism spectra should be compared with grating spectra, obtained in the same 
manner from the same elements, and attention may be directed to the repro- 
ductions of prism spectra of iron, nickel, cobalt, and gold.* Platinum was first 
published in the Journal of the Chemical Society in 1882.+ 
The production of powerful sparks by means of dynamos, producing alternating 
currents, has been one of the causes of the variation in the character of spark 
spectra, as for instance in the work of Liveing and Dewar, Trowbridge and 
Sabine, Kayser and Runge, Eder and Valenta, and by the use of special forms of 
induction coils by Eugene Demarcay de Gramont, also Exner and Haschek. The 
last named workers used a current transformer combined with a coil, such as is 
suitable for Tesla’s experiments. 
The Occurrence of Aw-lines in Spark Spectra. 
The method of photographing the spectra of metals without the usually 
accompanying spectrum of air, which has been particularly described by 
Hemsalach,t also gets rid of the short lmes and dots observed in many spectra, 
for instance in that of zinc. The process of self-induction, which is very simple, 
apparently lowers the temperature of the spark, and the spectra then more nearly 
approach the character of those obtained by the arc. 
As these photographs were taken before the method of Hemsalach was published, 
it was impossible to employ it; it is advisable, however, to state that the air lines 
serve to definitely fix the positions of neighbouring lines of other elements, and in 
certain parts of the spectrum they prove to be a great convenience. 
Numerous observations have also shown that, no matter how dense the spectrum 
* Sci. Trans. Roy. Dublin Soc., vol. i, ser. 2, p. 231, 1882. 
{ Journ. Chem. Soc., vol. xli., p, 84. 
t Comptes Rendus, vol. exxix., p. 285, 1899. 
