On the character and intensity of the rays emitted by glowing platinum

IN 1860, Kircbhoff* issued bis well-known paper on the relation between tbe capacity of bodies for emitting and for absorbing rays. That essay made a new epoch in the science of Radiation. It offered the first complete proof and the first universal expression of a principle which had existed in tbe minds of scientists, more or less dimly, since the days of Euler.t Although the results of that treatise have been repeatedly confirmed by the experience of investigators in Optical Science and in the domain of Radiant Heat, there have been, so far as I know, in spite of the interesting character of Kirchhoff's Function I,:!: no attempts to measure its val ues. * Kirchhoff, Poggendorff's Annalen, cix j also, "Untersnchnngen iiber das Sonnenspectrum-Anhang." t For earlier attempts to express what is now known as Kirchhoff's Law, see Euler, Opnscula Varii Argumenti, Berol. 1746 (Nova Theoria Lucia et Colorum, Cap. V). Pierre Prevo8t, Physische-mechanische Untersuchungen iiber die Warme, Halle, 1798. Angstrom, Pog-gendor:ff's Annalen, xciv., Balfonr Stewart, Proceedings of the Royal Society of Edinburgh, 1857·-58. * In the above-mentioned treatise Kirchhoff gives for I the following formula: