on a surface perpendicular to the direct rays.  This is equivalent to the illumination due to a source 90,000 candle-power at a distance of three feet.  The sun delivers about 200,000,000,000 horse-power to the earth continuously, which is estimated to be about one million times the amount of power generated artificially on the earth.  Of this inconceivable quantity of energy a small part is absorbed by vegetation, some is reflected and radiated back into space, and the balance heats the earth.  To store some of this energy so that it may be utilized at will in any desired form is one of the dreams of science.  However, artificial light-sources are depended upon at present in many photographic and other chemical processes.

Although two illuminants may be of the same luminous intensity, they may differ widely in actinic value.  It is impossible to rate the different illuminants in a general manner as to actinic value because the various photochemical reactions are not affected to the same extent by rays of a given wave-length.  Nearly all human eyes see visible rays in approximately the same manner, but the multitude of chemical reactions show a wide variation in sensitivity to the various rays.  For example, one photographic emulsion may be sensitive only to ultra-violet, violet, and blue rays and another to all these rays and also to the green, yellow, and red.  Therefore, one illuminant may be superior to another for one photochemical reaction, while the reverse may be true in the case of another reaction.  In general, it may be said that the arc-lamps including the mercury-arcs provide the most active illuminants for photochemical processes; however, a large number of electric incandescent filament lamps are used in photographic work.

The photo-engraver has been independent of sunlight since the practical development of his art.  In fact, the printer could not depend upon sunlight for making the engravings which are used to illustrate the magazines and newspapers.  The newspaper photographer may make a “flashlight” exposure, develop his negative, and make a print from it under artificial light.  He may turn this over to the photo-engraver who carries out his work by means of powerful arc-lamps and in an hour or two after the original exposure was made the newspaper containing the illustration is being sold on the streets.

The moving-picture studio is independent of daylight in indoor settings and there is a tendency toward the exclusive use of artificial light.  In this field mercury-vapor lamps, arc-lamps, and tungsten photographic lamps are used.  Similarly, in the portrait studio there is a tendency for the photographer to leave the skylighted upper floors and to utilize artificial light.  In this field the tungsten photographic lamp is gaining in popularity, owing to its simplicity and to other advantages.  Artificial light in general is more satisfactory than natural light for many kinds of photographic work because through the