420 
POPULAR SCIENCE REVIEW. 
SPECTRUM ANALYSIS.* 
HE demand for a reprint of this work within a few months of its first 
issue is, of itself, evidence of the interest which it has deservedly ex- 
cited. Nor is there any difficulty in explaining the fact, since it distinctly 
occupies a gap, and fills up a lacuna left by such excellent treatises as those 
of Roscoe and Schellen. It does not even clash with the same author’s 
other volume on working with the Spectroscope, published by Macmillan. 
The discovery of spectrum analysis has indeed opened such a vast and new 
field to human observation, that there is abundant room, nay, imperative 
necessity, for manuals and treatises of very diverse nature. This is well 
described in its opening sentence : a The work of the true man of science,” 
says the author, “ is a perpetual striving after a better and closer knowledge 
of the planet on which his lot is cast, and of the Universe in the vastness of 
which that planet is lost. The only way of doing this effectually, is to pro- 
ceed as gradually, and therefore as surely, as possible, along the dim un- 
trodden ground lying beyond the known.” 
Whereas former works have been dogmatic, descriptive, and educational, 
this rather aims at being tentative, inductive, and experimental. 
As the whole subject is to many readers new and unfamiliar, the first 
forty-six pages have been devoted to a precis of the great generalization of 
waves and wave motion on which modern physics rests. These we may be 
excused for passing over in a brief notice like the present. The second 
chapter deals with modes of demonstration and laboratory work. Here we 
at once find matter of specific interest. 
Attention is called to the difference between the ordinary spectroscope 
and one that has a lens before the slit. If light be allowed to enter indis- 
criminately as it emanates from a complex source, in each part of the spec- 
trum we get an “ integration ” of the light of the same wave-length pro- 
ceeding from all the different light- waves. But if by means of a lens we 
form an image of the source, so that each particular part shall be impressed 
in its proper place on the slit, then in the spectrum the different kinds of 
light will be sorted out, and if there be any variety in the phenomena which 
these different sources of light present, they will be all clearly shown in the 
spectrum or its photograph. We turn the spectroscope from an “Integrator” 
into an “ Analyser.” The slit for a vertical image of a candle, for instance, 
requires to be placed horizontally. We then get three perfectly distinct 
spectra indicating the coexistence of three light sources, each with its proper 
spectrum, in the light of a common candle. 
This mode of observation, originally proposed in 1866 by the author, for 
the examination of minute portions of the Sun and his surrounding atmo- 
sphere, has been adopted in most of the work subsequently detailed. In a 
photograph of a horizontal electric arc, thrown by means of a lens on a 
vertical slit, it is shown that the spectrum consists of a large number of 
lines of different lengths in the “ core ” or centre of the arc, all of which die 
* “ Studies on Spectrum Analysis.” By J. Norman Lockyer, F.R.S. Or. 
8 vo. pp. 258. Kegan Paul & Co, International Scientific Series. Second 
edition. 
