. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. RADIATION. 299 endless screw, tLis linear thermo-electric jnle may bo moved tlironirh tlie entire spectrum, each of its rays being selected in succession, the amount of heat fall- ing upon the pile at every point of its march being declared Ity an associated magnetic needle. When this instrument is brought up to the violet end of the spectrum of the electric light, the heat is found to be insensible. As the pile gradually moves from the vio
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. RADIATION. 299 endless screw, tLis linear thermo-electric jnle may bo moved tlironirh tlie entire spectrum, each of its rays being selected in succession, the amount of heat fall- ing upon the pile at every point of its march being declared Ity an associated magnetic needle. When this instrument is brought up to the violet end of the spectrum of the electric light, the heat is found to be insensible. As the pile gradually moves from the violet end towards the red, heat soon manifests itself, augmenting as we approach the red. Of all the colors of the visible spectrum the red possesses the highest heating power. On pushing the pile into the dark region beyond the red, the heat, instead of vanishing, rises sudd(!nly and enormously in inten- sity, until at some distance l)eyond the red it attains a maximum. Jiloving the pile still forward, the thermal power falls, somewhat more suddenly than it rose. It then gradually shades away, but for a distance beyond the red greater tlian the length of the whole visible spectrum, signs of heat may bo detected. Draw- ing, as Sir William Herschel did, a datum line, and erecting along it perpendic- ulars proportional in length to the thermal intensity at the respective points, we obtain the extraordinary curve which exhibits the distribution of heat in the spectrum of the electric light. In the region of dark rays beyond the red, the curve shoots np in a steep and massive peak—a kind of Matterhorn of heat, which dwarfs by its magnitude the portion of the diagram representing the luminous radiation. Indeed, the idea forced upon the mind by the inspection of this diagram is that the light rays are a mere insigniticant appendage to the dark ones, thrown in as it were by nature for the purposes of vision. (See figure, where the space ABCD represeuta the non-luminous, and CBE the lu
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