THE LIGHT-QUANTUM 367Sample of the Rubens-Kurlbaum data which led Planck to guess his radiation formula
[Rl]. P is plotted versus Tfor X = 51.2/un. ("berechnet nach" means "computed after",
"beobachtet" means "observed".) The curves marked "Wien" and "Lord Rayleigh" refer
to best fits to the Eqs. (19.5), (19.17), respectively. The curves marked "Thiesen" and
"Lummer-Jahnke" refer to theoretical proposals which are not discussed in this book.
Planck's formula is not yet plotted.
The paper by Rubens and Kurlbaum was presented to the Prussian Academy
on October 25, 1900. The figure above shows some of the measured points they
recorded* and some theoretical curves with which they compared their findings.
One of these was the Wien curve, which did not work. Neither did a second curve,
proposed by Rayleigh (I shall return to Rayleigh's work in section 19c). I shall
leave aside the two other comparison curves they drew and turn to the all-impor-
tant 'fifth formula, given by Herr M. Planck after our experiments had already'These refer to observations at X = 51.2f«n. This wavelength was isolated by multiple reflections
off rock salt. The blackbody radiation intensity is plotted as a function of T. (Recall that after mul-
tiple reflection, those specific frequencies predominantly survive which correspond to the ionic vibra-
tions in the crystal lattice chosen as reflector.)