THE old saying that small causes give rise to great effects has been confirmed more than once in the history of physics. For, very frequently, inconspicuous differences between theory and experiment (which did not, however, escape the vigilant eye of the investigator) have become starting points of new and important researches. Out of the well-known Michelson-Morley experiment, which, in spite of the application of the most powerful methods of exact optical measurement, failed to show an influence of the earth's movement on the propagation of light as was predicted by classical theory, there arose the great structure of Einstein's Theory of Relativity. In the same way the trifling difference between the measured and calculated values of black-body radiation gave rise to the Quantum Theory which, formulated by Max Planck, was destined to revolutionise in the course of time almost all departments of physics. THE quantum theory is yet comparatively young. It is therefore not surprising that we are confronted with an unfinished theory still in process of development which, changing constantly in many directions, must often destroy what it has built up a short time before. But under such circumstances as these, in which the theory is continually deriving new nourishment from a fresh stream of ideas and suggestions, there is a peculiar fascination in attempting to review the life-history of the quantum theory to the present time and in disclosing the kernel which will certainly out-last changes of form. THE Quantum Theory first saw light in 1900. When, in the years immediately preceding (1897-1899), Lummer and Pringsheim made their fundamental measurements of black-body radiation at the Reichsanstalt, they could have had no premonition that their careful experiments would become the starting-pointof a revolution such as has seldom occurred in physics. In the field of heat radiation chief interest at that time was centred in the radiation of a black body (briefly called " blackbody " radiation), that is, of a body which absorbs completely all radiation which falls on it and which thus reflects, transmits, and scatters none. We may shortly call to mind the following facts. It is known that any body at a given temperature sends out energy in the form of radiation into the surrounding space. This radiation is not energy in a single simple form but is made up of a number of single radiations of different colours, i.e. of different wave-lengths (Lambda) or of different frequencies.