222 ANNUAL BEPOKT SMITHSONIAN INSTITUTION, 19 30 



By the application of thermodynamics to radiation processes 

 Boltzmann proved that the total radiation, of all wave lengths, 

 within a cavity in a heated body must increase in proportion to the 

 fourth power of the absolute temperature; this law had already 

 been found empirically by Stefan. By a further development of 

 thermodynamic methods Wien, in 1896, derived an important law, 

 known as Wien's law, b}^ which the intensity of radiation of any 

 particular wave length could be calculated in terms of the wave 

 length and temperature. This law was found to agree with experi- 

 ment in the case of visible radiation from incandescent solids, but 

 serious discrepancies were observed when an attempt was made to 

 calculate the intensity of infra-red radiation or heat waves. Lord 

 Rayleigh and Jeans, in 1900, using what seemed to be unimpeachable 

 methods based on the electromagnetic theory of light, arrived at an 

 entirely different relation between the intensity of radiation and 

 the temperature and wave length. This equation agreed excellently 

 with experiments on the radiation of heat where Wien's law had 

 failed but led to absurd results when applied to the shorter wave 

 lengths of the visible spectrum. In fact, if the total radiation in- 

 cluding all wave lengths were calculated from the Rayleigh-Jeans 

 equations an infinite radiation density was obtained even at low 

 temperatures. Thus by means of the classical theories of radiation 

 it was found on the one hand by Boltzmann that the radiation in- 

 creased with the fourth power of the temperature, and on the other 

 by Rayleigh-Jeans, that the radiation was infinite at all temperatures. 



It was shown in 1905, by Planck, that this paradox could only 

 be solved by assuming an essential discontinuity in the energies or 

 motions of electrons whose vibrations caused the radiation. This 

 gave birth to the quantum theory, which within recent years has 

 grown to be one of the most important theories of physics and 

 chemistry. In 1906, Einstein showed that the photo-electric effect 

 and many photochemical reactions could be explained in terms of 

 the quantum theory if light itself consisted of discrete particles of 

 energy or quanta, now usually called photons. Although such a 

 corpuscular theory of light seemed utterly incompatible with the 

 accepted wave theory, an increasing number of phenomena were 

 discovered in which it seemed necessary to resort to this corpuscular 

 theory. The really rapid development of the quantum theory, how- 

 ever, dates from 1913, when Bohr began to develop his theory of 

 atomic structure by applying the quantum theory to Rutherford's 

 more or less qualitative theory of the nuclear atom. 



RELATIVITY THEORY 



Among all the changes in the ways of thinking which were being 

 forced upon physicists at this time, the most important by far was 



