Mr. Hewitt chose his problem deliberately, and he chose one of the most difficult in all the range of electrical science, but one which, if solved, promised the most flattering rewards. "The essence of modern invention," he said, "is the saving of waste, the increase of efficiency in the various mechanical appliances." This being so, he chose the most wasteful, the least efficient of all widely used electrical devices--the incandescent lamp. Of all the power used in producing the glowing filament in the Edison bulb, about ninety-seven per cent. is absolutely wasted, only three per cent. appearing in light. This three per cent. efficiency of the incandescent lamp compares very unfavourably, indeed, with the forty per cent. efficiency of the gasoline engine, the twenty-two per cent. efficiency of the marine engine, and the ninety per cent. efficiency of the dynamo. [Illustration: The Hewitt Mercury Vapour Light. _The circular piece just above the switch button is one form of "boosting coil" which operates for a fraction of a second when the current is first turned on. The tube shown here is about an inch in diameter and several feet long. Various shapes may be used. Unless broken, the tubes never need renewal._] Mr. Hewitt first stated his problem very accurately. The waste of power in the incandescent lamp is known to be due largely to the conversion of a considerable part of the electricity used into useless heat. An electric-lamp bulb feels hot to the hand. It was therefore necessary to produce a _cool light_; that is, a light in which the energy was converted wholly or largely into light rays and not into heat rays. This, indeed, has long been one of the chief goals of ambition among inventors. Mr. Hewitt turned his attention to the gases. Why could not some incandescent gas be made to yield the much desired light without heat? This was the germ of the idea. Comparatively little was known of the action of electricity in passing through the various gases, though the problem involved had long been the subject of experiment, and Mr. Hewitt found himself at once in a maze of unsolved problems and difficulties. "I tried many different gases," he said, "and found that some of them gave good results--nitrogen, for instance--but many of them produced too much heat and presented other difficulties." Finally, he took up experiments with mercury confined in a tube from which the air had been exhausted. The mercury arc, as it is called, had been experimented with years before, had even been used as a light, although at the time he began his investigations Mr. Hewitt knew nothing of these earlier investigations. He used ordinary glass vacuum tubes with a little mercury in the bottom which he had reduced to a gas or vapour under the influence of heat or by a strong current of electricity. He found it a rocky experimental road; he has called invention "systematic guessing."