Neither of them give us any information, however, {51} about the actual distribution of energy at any one temperature from which we may calculate that at any other temperature. For that, some relation must be found between the energy and the wave-length. Planck, by reasoning founded on the electromagnetic character of the waves, derived such a relation, but both his reasoning and his results are a little too complicated to be introduced here. His results have been confirmed in the most striking manner by experiments carried out by Rubens and Kurlbaum (_Ann. der Physik_, 4, p. 649, 1901). They measured the energy in a particular wave-length (.0051 cms., _i.e._ nearly 100 times the wave-length of red light) in the radiation of a full radiator from a temperature of 85° up to 1773° absolute, and their results are given in the following table: Absolute Temperature. Observed Energy. Energy calculated from Planck's Formula. 85 -20.6 -21.9 193 -11.8 -12 293 0 0 523 +31 +30.4 773 64.6 63.8 1023 98.1 97.2 1273 132 132 1523 164 160 1773 196 200 We have therefore the means of calculating both the total quantity and the kind of radiation given out by any full radiator at any temperature, and a number of very interesting problems may be solved by means of the results. {52}