There was a fundamental mistake…namely, the paying of attention solely to the result given by experimental measurements and entirely ignoring the inner realities of the inner processes.
Kirchhoff's Law of Thermal Emission was formulated in 1859 [Poggendorfs Annalen der Physik und Chemie, 1860, v. 109, 275–301; also in English - Phil. Mag., 1860, ser. 4, v. 20, 1–21]. It is at the same time the simplest and least understood law in physics. Kirchhoff’s law states that given thermal equilibrium with an enclosure, the radiation inside will be always black, or normal, in a manner which is independent of the nature of the walls, or the objects they contain. This is known as the concept of universality. That is, that the radiation within an enclosure can always be described by a universal function dependent only on temperature and frequency. This universal function was first given to us by Max Planck, in 1900 [Annalen der Physik, 1901, v. 4, 553–563]. Kirchhoff’s law stands at the heart of all of modern astrophysics. It is the basis for setting the temperature of the stars, for the gaseous model of the Sun, and for believing that we now know the temperature of the entire universe.
Tragically, Kirchhoff’s belief that radiation within an enclosure was independent of the nature of the walls is not correct. Kirchhoff arrived at universality because he did not properly address reflection within cavities. As a result, the proper law of thermal emission was brought to us by Balfour Stewart, in 1858 [Trans. Royal Soc. Edinburgh., 1858, v. 22(1), 1–20]. Like Kirchhoff’s law, Stewart’s law properly equates the emissivity and absorptivity of an object at thermal equilibrium, but it does not lead to universality. Stewart’s law states that the radiation is determined not only by temperature and frequency, but also by the nature of the walls themselves.
P-M. L. Robitaille. On the validity of Kirchhoff’s law of thermal emission. IEEE Trans. Plasma Sci., 2003, v. 31(6), 1263–1267.