When this instrument is brought to the violet end of the spectrum, the heat is found to be almost insensible.  As the pile gradually moves from the violet towards the red, it encounters a gradually augmenting heat.  The red itself possesses the highest heating power of all the colours of the spectrum.  Pushing the pile into the dark space beyond the red, the heat rises suddenly in intensity, and at some distance beyond the red it attains a maximum.  From this point the heat falls somewhat more rapidly than it rose, and afterwards gradually fades away.

Drawing a horizontal line to represent the length of 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.