Photometry and the testing of incandescent lamps

At one time photometry was regarded merely as an interesting and fascinating study for the professor in the laboratory. To-day it is a process of industrial importance. New illuminants have come to the fore and new methods of employing them are constantly being devised — and how can we compare their merits without measuring the light derived from them? Whatever be the method of lighting adopted, it is the illumination derived from it with which we are concerned and for which we pay. Measurement of this commodity would therefore seem to be an elementary safeguard to be observed in any contract relating to the sale of light.