Experimental techniques had sufficiently advanced by then to put this formula to
the test. This was done by Friedrich Paschen from Hannover, whose measure-
ments (very good ones) were made in the near infrared, X = 1-8 fim (and T =
400-1600 K). He published his data in January 1897. His conclusion: 'It would
seem very difficult to find another function [of v and T, Eq. 19.5] that represents
the data with as few constants' [PI]. For a brief period, it appeared that Wien's
law was the final answer. But then, in the year 1900, this conclusion turned out
to be premature and the correct response to Kirchhoff's challenge was found. Two
factors were decisive. One was a breakthrough in experimental techniques in the
far infrared. The other was the persistence and vision of Planck.
It happened in Berlin. At the Physikalisch Technische Reichsanstalt, at that
time probably the world's best-equipped physics laboratory, two teams were
independently at work on blackbody radiation experiments. The first of these,
Otto Lummer and Ernst Pringsheim, had tackled the problem in an as yet unex-
plored wavelength region, X = 12-18 /an (and T = 300-1650 K). In February
1900 they stated their conclusion: Wien's law fails in that region [LI].* The sec-
ond team, consisting of Heinrich Rubens and Ferdinand Kurlbaum, moved even
farther into the infrared: X = 30-60 urn (and T = 200-1500° C). They arrived
at the same conclusion [Rl].
I need to say more about the latter results, but I should like to comment first
on the role of experiment in the discovery of the quantum theory. The Rubens-
Kurlbaum paper is a classic. The work of these authors, as well as that of Paschen
and of Lummer and Pringsheim, was of a pioneering nature. By the middle of the
nineteenth century, wavelengths had been measured up to X « 1.5/im. Progress
was slow in the next forty years, as demonstrated by a question raised by Samuel
Pierpont Langley in a lecture given in 1885 before the AAAS meeting in Ann
Arbor: 'Does [the] ultimate wavelength of 2.7 pm which our atmosphere transmits
correspond to the lowest [frequency] which can be obtained from any terrestrial
source?' [L2]. The great advance came in the 1890s. The first sentence of the first
paper in the first issue of the Physical Review reads as follows: 'Within a few
years the study of obscure radiation has been greatly advanced by systematic
inquiry into the laws of dispersion of the infrared rays.' This was written in 1893,
by Ernest Fox Nichols. At about that time, new techniques were developed which
culminated in the 'Reststrahlen,' residual rays, method of Rubens and Nichols
[R2]: one eliminates short wavelengths from a beam of radiation by subjecting it
to numerous reflections on quartz or other surfaces. This procedure leads to the
isolation of the long wavelengths in the beam. These experimental developments
are of fundamental importance for our main subject, the quantum theory, since
they were crucial to the discovery of the blackbody radiation law.
* There had been earlier indications of deviations from Wien's law, but these were not well
documented.366 THE QUANTUM THEORY(19.5)