1874.] Notices of Books. 103 
spectrum, which we now call “ Fraiinhofer’s lines.” Ten years 
later, in 1812, the German optician, Fraiinhofer mapped no less 
than 576 of these lines, and lettered the principal ones A, B, C, 
D; he discovered, moreover, that in the spectrum of certain 
stars, the black lines do not hold by any means the same position 
which they hold in the spectrum of the sun. In 1830, the next 
great improvement in the spectroscope was made. In addition 
to the simple prism and slit, Mr. Simms, the optician, placed a 
lens in front of the prism and another lens near the eye, so as 
to magnify the spectrum. By this means the black lines could 
be studied with far greater readiness than by the naked eye. 
“You may imagine the enormous mystery—the wonderful 
reverence almost—with which this question of the Fraiinhofer’s 
lines was approached until they were thoroughly understood ; 
and recollect that we owe the discovery of them, by which we 
are enabled now to determine the pressures acting in the atmo- 
spheres of the most distant stars, simply to the fact that Dr. 
Wollaston, instead of drilling a round hole, used a slit; and to 
the other additional fact, that Mr. Simms, instead of using that 
slit with a mere prism, used a lens, and made the beam parallel, 
and then aliowed that parallel beam, after it had passed through 
the prism, to pass into another telescope, and form an image of 
the slit for each ray. You see how closely connected are the 
grandest discoveries with the skill and suggestiveness of those 
who supply different instruments for our use.” Thus the principal 
incidents in the history of the spectroscope, as an instrument, 
are (a) Newton’s application of the prism to optical purposes, in 
1675; (8) his observation that the prism should be used at the 
angle of minimum deviation; (y) the addition of the slit by 
Wollaston in 1812; (6) the collimating lens added by Simms in 
1830. The author next describes various forms of spectroscopes 
with one, two, or more prisms. Capital figures are given of 
Steinheil’s four-prism spectroscope, used by Kirchhoff; of 
Huggins’s star spectroscope, and of direct vision spectroscopes 
with three or four prisms. 
The second lecture treats of the applications of the spectro- 
scope, specially of those which depend upon the investigation of 
light radiated from bodies. Thus applied, the instrument enables 
us to distinguish between solids, liquids, and gases; and between 
gases and vapours existing at different pressures. The various 
methods of obtaining spectra are here discussed; the use of the 
Bunsen burner, the induction coil, and the voltaic arc. A coloured 
plate at the commencement of the volume shows, among other 
things, two spectra of great interest; the one of hydrogen at a 
high pressure, the other of hydrogen at a low pressure. It is 
here seen that, in the former instance, the spectrum is much 
more continuous than in the latter. In fact, in the vacuous 
tube, the hydrogen spectrum dwindles down to one or two very 
sharply defined and thin lines. On the other hand, Frankland 
