A Brief History Of The T⁴ Radiation Law

Planck (1900), Improvement of Wien's Law

Planck (1900), On the Theory of the Energy Distribution Law of the Normal Spectrum

Planck (1901), On the Law of the Energy Distribution in the Normal Spectrum

How does a continuous black-body spectrum emerges from discrete energy gases (blog)

Hot gases: The transition from line spectra to thermal radiation - Vollmer 2005 (contains a critical error, but the general idea is well explained)

Comment on "Hot gases: The transition from line spectra to thermal radiation" + correct derivation of the emergence of black-body spectrum from gases - Nauenberg, 2007