"The inverse of the process of producing light quanta by the impact of electrons is the emission of electrons on exposure to light-rays, or s, and here... energy quanta following from the action quantum and the vibration period play a characteristic role, as was early recognized from the striking fact that the velocity of the emitted electrons depends not upon the intensity but only on the colour of the impinging light. [T]the relations to the light quantum, pointed out by Einstein, have proved successful in every direction... shown especially by R. A. Millikan, by measurements of the velocities of emission of electrons, while the importance of the light quantum in inducing photo-chemical reactions was disclosed by E. Warburg."