IN any branch of engineering, practice usually precedes theory. As practice develops and becomes more wide-spread,its faults show themselves, and in the attempt to remedy these faults the theory is evolved. Theory reacts upon practice, improving it, and practice in turn reacts upon theory, causing a development of shorter methods of predetermining results.
Thus, in the field of illumination practice is several thousand years old, but the theory is only beginning to be developed. The development of the early forms of illumination was not accompanied by any great increase in intrinsic brilliancy or intensity of light per square inch of lighting surface. The fire brand of the savage naturally led to the use of the pine torch for illumination. "]'his in turn was succeeded by candles and oil lamps. Until the introduction of illuminants more efficient than these, the intrinsic brilliancy of the light-source remained very low. The quantity of light which could be obtained from a single light-source also remained low, and to illuminate large areas large numbers of light-sources were necessary. It was a simple matter to place these lights where desired. With the development of gas and electric lights, whose locations are more or less permanent, faults of location began to be apparent. Candles or oil lamps could be moved to the part of the room in which the light was wanted. For example in a bed room they could be placed near the mirror when it was in use. Generally speaking this is impracticable with gas and electricity, however, and in order to obtain the light at the proper points, it is necessary to either flood the room with light or to locate the lights where light is desired. The former is undesirable from the standpoint of economy, and hence arose the study of light location as a part of the work of the designer 362 " The Calculation of Illumination by the Flux of Light Method"--