the spectrum, and
erecting along it, at various points, perpendiculars proportional in
length to the heat existing at those points, we obtain a curve which
exhibits the distribution of heat in the prismatic spectrum. It is
represented in the adjacent figure. Beginning at the blue, the curve
rises, at first very gradually; towards the red it rises more rapidly,
the line C D (fig. 54, opposite page) representing the strength of the
extreme red radiation. Beyond the red it shoots upwards in a steep and
massive peak to B; whence it falls, rapidly for a time, and afterwards
gradually fades from the perception of the pile. This figure is the
result of more than twelve careful series of measurements, from each
of which the curve was constructed. On superposing all these curves, a
satisfactory agreement was found to exist between them. So that it may
safely be concluded that the areas of the dark and white spaces,
respectively, represent the relative energies of the visible and
invisible radiation. The one is 7.7 times the other.

But in verification, as already stated, consists the strength of
science. Determining in the first place the total emission from the
electric lamp, and then, by means of the iodine filter, determining
the ultra-red emission; the difference between both gives the luminous
emission. In this way, it is found that the energy of the invisible
emission is eight times that of the visible. No two methods could be
more opposed to each other, and hardly any two results could better
harmonize. I think, therefore, you may rely upon the accuracy of the
distribution of heat here assigned to the prismatic spectrum of the
electric light. There is nothing vague in the mode of investigation,
or doubtful in its conclusions. Spectra are, however, formed by
_diffraction_, wherein the distribution of both heat and light is
different from that produced by the prism. These diffractive spectra
have been examined with great skill by Draper and Langley. In the
prismatic spectrum the less refrangible rays are compressed into a
much smaller space than in the diffraction spectrum.

LECTURE VI.

PRINCIPLES OF SPECTRUM ANALYSIS
PRISMATIC ANALYSIS OF THE LIGHT OF INCANDESCENT VAPOURS
DISCONTINUOUS SPECTRA
SPECTRUM BANDS PROVED BY BUNSEN AND KIRCHHOFF TO BE CHARACTERISTIC
OF THE VAPOUR
DISCOVERY OF RUBIDIUM, CAESIUM, AND THALLIUM
RELATION OF EMISSION TO ABSORPTION
THE LINES OF FRAUNHOFER
THEIR EXPLANATION BY KIRCHHOFF