The short exposure required in portrait and motion-picture photography demands a high intensity of actinic light. At the same time, in order that the reproduction of facial expression may be natural, the brightness o,f t;he light should not .be so intense as to cause the subject to squint. These contrary requirements led to the development of a special blue-grass bulb for IOOO-watt Mazda C. lamps, which is known as the " photographicblue" bulb. This ,bulb screens out abou,t two-thirds of the light emitted by a Mazda C. lamp filament, but it transmits, as shown by spectograms, all those rays which are actinic to those green-and yellow-sensitive emulsions that are usually called orthochromatic. Furthermore, the light from the photographic-blue bulb Mazda C. lamp appears like daylight, and its ordinary-emulsion actinicity per candle-power is so nearly equal ,to that of daylight that it may be mixed with daylight without any allowance being made in judging the proper time of exposure. This is an important quality, for it enables the photographer to .estimate amounts of exposure by the brightness of the subject in accordance with his usual practice. Obviously, these advantages, namely, the reduction in brightness and likeness .to daylight, can be utilized to the fullest extent wi.th ordinary emulsions, and, since most portrait and motion-picture photographs are .made on such emulsions, the photographic-blue bulb is particularly adapted to the illumination of .the usual studio.