in which is tine energy absorbed by the electron from the light wave or light quantum, for, according to the assumption it was the whole energy contained in that quantum, is the work necessary to get the electron out of the metal, and is the energy with which it leaves the surface—an energy evidently measured by the product of its charge by the potential difference against which it is just able to drive itself before being brought to rest. At the time at which it was made this prediction was as bold as the hypothesis which suggested it, for at that time there were available no experiments whatever for determining anything about how the positive potential necessary to apply to the illuminated electrode to stop the discharge of negative electrons from it under the influence of monochromatic light varied with the frequency of the light, or whether the quantity to which Planck had already assigned a numerical value appeared at all in connection with photo-electric discharge. We are confronted, however, by the astonishing situation [Pg 239] that after ten years of work at the Ryerson Laboratory (1904-15) and elsewhere upon the discharge of electrons by light this equation of Einstein’s was found to predict accurately all of the facts which had been observed. IV. THE TESTING OF EINSTEIN’S EQUATION The method which was adopted in the Ryerson Laboratory for testing the correctness of Einstein’s equation involved the performance of so many operations upon the highly inflammable alkali metals in a vessel which was freed from the presence of all gases that it is not inappropriate to describe the experimental arrangement as a machine-shop in vacuo. Fig. 32 shows a photograph of the apparatus, and Fig. 33 is a drawing of a section which should make the necessary operations intelligible.