In the last four months of 1859, there occurred a number of events that were to change the course of science.
On the twelfth of September, Le Verrier submitted to the French Academy of Sciences the text of his letter to Faye concerning an unexplained advance of the perihelion of Mercury (see Section 14c), the effect explained by Einstein in November 1915. On the twenty-fourth of November, a book was published in London entitled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, by Charles Robert Darwin . Meanwhile on the twentieth of October, Gustav Kirchhoff from Heidelberg submitted his observation that the dark D lines in the solar spectrum are darkened still further by the interposition of a sodium flame [K1]. As a result, a few weeks later he proved a theorem and posed a challenge. The response to Kirchhoff's challenge led to the discovery of the quantum theory.
Consider a body in thermal equilibrium with radiation. Let the radiation energy which the body absorbs be converted to thermal energy only, not to any other energy form. Let Evdv denote the amount of energy emitted by the body per unit time per cm2 in the frequency interval dv. Let Av, be its absorption coefficient for frequency v. Kirchhoff's theorem [K2] states that Ev/Av depends only on v and the temperature T and is independent of any other characteristic of the body:
Ev/Av = J(v, T) (19.1)
Kirchhoff called a body perfectly black if Av = 1. Thus J(v, T) is the emissive power of a blackbody. He also gave an operational definition for a system, the 'Hohlraumstrahlung,' which acts as a perfect blackbody: 'Given a space enclosed by bodies of equal temperature, through which no radiation can penetrate, then every bundle of radiation within this space is constituted, with respect to quality and intensity, as if it came from a completely black body of the same temperature.'
Kirchhoff challenged theorists and experimentalists alike: "It is a highly important task to find this function [J]. Great difficulties stand in the way of its experimental determination. Nevertheless, there appear grounds for the hope that it has