120 IOWA ACADEMY OF SCIENCE 



Having determined the intensity of illumination required; the next 

 problem is to select the illuminant which will give most nearly the 

 desired effect. As the effect is determined as much by color as by in- 

 tensity and color can be modified by the use of colored globes, the 

 choice of an illuminant for a given case is determined largely by avail- 

 ability and convenience. Owing to the almost universal use of electric 

 power and the great variety and convenience of electric lamps they are 

 usually given the preference, though they are surpassed in cheapness 

 by gas and oil lamps. The intensity of illumination produced by a 

 given set of light sources may be calculated exactly; so this part of 

 the problem is comparatively simple, except as architectural features 

 may interfere with the desired location of the lamps. 



This brings up the third part of the problem; the relation of illu- 

 mination and lamp supports to architecture. A room may be made or 

 marred, artistically, by either the lamp supports or the illumination. 

 No doubt each of you can recall an instance of a room which was har- 

 monious as to every decoration but which was equiped with lamp fix- 

 tures which did not match the decorations and so spoiled an otherwise 

 pleasing effect ; or a room which was well proportioned by daylight but 

 which looked all out of proportion by artificial light, due to the effect 

 of the change in the position of the shadows. The lamps and their sup- 

 ports should be such that the occupants of a room are not conscious of 

 them except as part of the general scheme of decoration. 



Fourth and last the installation must be economical in first cost and 

 operation. This does not necessarily mean that it must be cheap, for 

 in many cases cheapness is not economy but waste ; as in the case of a 

 poorly lighted shop where the decreased efficiency of the workmen, due 

 to the poor light, involves a greater expense, per unit of product, than 

 an adequate lighting system. 



The materials with which the illuminating engineer deals, lamps of 

 various kinds, have been studied longer than either the psycho-physi- 

 ological or the architectural sides of the profession and are less vari- 

 able ; so their characteristics are well known. Lamps may be roughly 

 divided into open flame lamps (torches, oil lamps, open gas flames) ; 

 flame heated incandescent lamps (mantle gas and oil lamps) ; electrically 

 heated incandescent lamps (carbon and tungsten electric lamps) ; and 

 electric arcs (carbon arcs, metallic arcs, flame arcs). Of these the 

 electric arcs are the most efficient; though the best of them have an 

 efficiency of only about 15%, which is about 2.5 times that of the 

 tungsten incandescent lamp and 150 times that of the mantle gas lamp. 



While illuminating engineering has been done in the past and is 



