In the first place, it has been proved that in factories proper lighting as obtained by artificial means is generally more satisfactory than the natural lighting. Of course, a narrow building with windows on two sides or a one-story building with a saw-tooth roof of best design may be adequately illuminated by natural light, but these buildings are the exception and they will grow rarer as industrial districts become more congested. Artificial light may be controlled so that light of a satisfactory quality is properly directed and diffused. Sufficient intensities of illumination may be obtained and the failure of artificial light is a remote possibility as compared with the daily failure of natural light. With increasing cost of ground space, factories are built of several stories and with less space given to light courts, with the result that the ratio of window area to that of the floor is reduced. These tendencies militate against satisfactory daylighting. In the smoky congested industrial districts the period of effective daylight is gradually diminishing and artificial lighting is always essential at least as a reinforcement for daylight. It has been proved that proper artificial lighting--and there is no excuse for improper artificial lighting--is superior to most interior daylighting conditions. [Illustration: LOCOMOTIVE ELECTRIC HEADLIGHT] [Illustration: SEARCH-LIGHT ON A FIRE-BOAT] [Illustration: BUILDING SHIPS UNDER ARTIFICIAL LIGHT AT HOG ISLAND SHIPYARD] Although it is difficult to present figures in a brief discussion of this character, it may be stated that, in general, the cost of adequate artificial light is about 2 per cent. of the pay-roll of the workers; about 10 per cent. of the rental charges; and only a fraction of 1 per cent. of the cost of the manufactured products. These figures vary considerably, but they represent conservative average estimates. From these it is seen that artificial lighting is a small factor in adding to the cost of the product. But does artificial lighting add to the cost of a product? Many examples could be cited to prove that proper artificial lighting may be responsible for an actual reduction in the cost of the product. In a certain plant it was determined that the workmen each lost an appreciable part of an hour per day because of inadequate lighting. A properly designed and maintained lighting-system was installed and the saving in the wages previously lost, more than covered the operating-expense of the artificial lighting. Besides really costing the manufacturer less than nothing, the new artificial lighting system was responsible for better products, decreased spoilage, minimized accidents, and generally elevated spirits of the workmen. In some cases it is only necessary to save one minute per hour per workman to offset entirely the cost of lighting. The foregoing and many other examples illustrate the insignificance of the cost of lighting.