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.