XVI.—PHOTOGRAPHS OF THE SPARK SPECTRA OF TWENTY-ONE 
ELEMENTARY SUBSTANCES. By W. N. Harruny, r.r.s.z., Proressor 
oF Cuemistry, Royan Coiuean or Scrunce, Dusuin.—Puates XVI, XVII., 
anp XVIII. 
Read April 11th, 1881. 
I 
Tux spectra of the elements are physical constants of the greatest importance, 
which have hitherto been but imperfectly studied. Those spectra which have 
received most attention have included but little more than rays of such refrangi- 
bility as are plainly visible. The time occupied and the labour involved in making 
accurate observations with the ordinary forms of spectroscopic apparatus have, to 
a certain extent, been a bar to our knowledge of the conditions under which the 
elements emit characteristic spectra, and have precluded the useful applications 
of spectroscopy to the technical purposes of the metallurgist and chemist. Un- 
doubtedly at the present time more reliance is to be placed on the ordinary 
methods of chemical analysis than on those dependent on the use of the spectro- 
scope ; there is reason to believe, however, that further research, particularly in the 
branch of spectrum-photography, will lead to greater refinements and precision both 
in qualitative and quantitative analysis than have hitherto been attained. * 
The character of a photographed spectrum depends on the nature of the salt or 
salts in and on the surface of the sensitive film of the photographic plate, the 
length of period of exposure of the film to the active rays, the diactinic character 
of the optical train, and the rates of vibration of the emitted rays. Given an 
optical train as diactinic as air, and a sensitive salt capable of receiving an 
impression from all rays transmitted by the instrument, and we may regard a 
photograph of a spectrum as a permanent record of a chemical reaction distributed 
in time and space. 
In presenting the accompanying photographs of spectra, which were obtained by 
means of the apparatus and processes previously described by me (Proc. R. D. 
Soc., Vol. 3, New Series), I am desirous, of making known the excellence of this 
method of research and of recording for future reference, the number, the relative 
positions, and what I may term the graphic peculiarities of the rays emitted by 
various substances. 
On the graphic characters of spectral lines.—The characters of the ultra-violet 
spectra are more strongly marked than those of the visible region, and the 
grouping of lines is more plainly seen by reason of the small number of air lines 
*See “On the Application of the Spectroscope to the Analysis of Iron and Steel.” By John 
Parry, F.c.s, and Alex. E, Tucker, F.c.s., ‘‘ Engineering,” February 14th, 1879. 
TRANS, ROY, DUR. SOC., N.S., VOL, I. 4.8 
