Gustav Kirchhoff

“His papers and lectures... form one of the enduring monuments in physical science.”

“His contributions extend over optics, heat, fluid, motion, electricity, elasticity, etc., and all bear the imprint of the great genius...”

“In 1875 he accepted... the chair of {{w|theoretical physics}} at Berlin where he became associated with his former colleague von Helmholtz.”

“In 1854 he... became associated with Bunsen. ...[H]e ...for twenty years ...in connection with Bunsen achieved some of the most important discoveries in the history of physical science.”

“By 1845 he had investigated electric currents, and established the two so-called Kirchhoff's laws for current conduction.”

“The formulation of the complete law for radiations of a {{w|Black body}} is only given in part by Kirchhoff. The formula of Wien, and more particularly the most recent one of Planck, deduced on theoretical grounds, approximates closely the latest observations on a black body at different temperatures and over different wave lengths.”

“[M]ost radiations from gases are not exclusively thermal... [T]he substances, cited by Kirchhoff and Bunsen, also give off... chemical.., electrical and fluorescent radiations which Kirchhoff excluded in the proof of his law.”

“We owe to Kirchhoff..., the first rigorous proof of the celebrated law (...Kirchhoff's law) of the emission and absorption of light and heat, and the application of the same by both Kirchhoff and Bunsen to Spectrum Analysis. The radiation of solids and liquids and gases follows the law exactly when the conditions upon which he founded it are rigorously fulfilled, namely, the complete transformation from one to the other of radiant energy and their intrinsic {{w|heat}}.”

“Why do you want to come into physics? All is done and understood.”

“The proof of this special law is similar to that of the general law, but simpler; it will therefore facilitate the understanding of the latter. Moreover, conclusions which are drawn from the special law will be used in the proof of the common law.”

“This law will be proven, first, for the case where only black bodies are compared with each other, that is, those whose absorptic power = 1; i.e., it will be shown that the radiating power of all black bodies is the same at the same temperature.”

“Under these conditions the following law holds : The ratio between the emissive and the absorptive power is the same for all bodies at the same temperature.”

“Conversely, upon the body there falls through the openings 2 and 1 a pencil of rays having the wave length \lambda, polarized in the plane a; of this, the body absorbs a part while it reflects or transmits the remainder; let the ratio of the intensity of the absorbed rays to the incident rays be A and let this be called the absorptive power of the body... The quantities E and A depend upon the nature of the condition of the body.., also upon the form and position of the openings.., the wave length \lambda and the direction of the plane a.”

“The quantity E is called the emissive power of the body.”

“Let E \partial \lambda be the intensity of the component polarized in a, or... [equivalently] the increase, which the vis viva of the ether beyond the screen S2 experiences through this component in the unit of time.”

“Through these openings passes a pencil of rays sent out by the body... consider the part [of the pencil], whose {{w|wave length}} lies between \lambda and \lambda + \partial \lambda, and let this be divided into two polarized components, whose planes of polarization are the [perpendicular] planes a and b... passing through the axis of the ray pencil.”

“Before a body... imagine two screens, S1 and S2 placed in which are two openings 1 and 2, whose dimensions are infinitely small with respect to their distance apart, and each of which has a center.”

“A perfect mirror, like every diathermanous body, can itself send out no rays; for if it did (confined in an enclosure of like temperature) it would warm this enclosure... and cool itself more and more.”

“[I]t will be assumed that perfectly diathermanous bodies are conceivable, that is, such which will absorb none of the incident heat rays of whatever nature these may be, and finally, that a perfect mirror is conceivable, i.e., a body which reflects completely all heat rays.”

“[R]adiation in empty space will be investigated..⇒ the [associated] black bodies must have a refracted index which differs infinitely little from 1.”