GASLIGHT, PORTRAITURE BY The use of gaslight is possible for portraiture. With ordinary gaslight (without mantles), ex posures are long even under the most favour able conditions, the light being poor in violet rays and comparativelAnon-actinic. Gaslight has advantages for isochromatic work, as without a yellow screen it gives practically the same result as daylight with a screen, isochromatic plates being used in both cases. With a very rapid plate, and the lens stopped down to f/8, the neces sary exposure with two gaslights (not incan descent) 3 ft. from the sitter would be about two minutes. Particular care is necessary in posing, using a reflector, and in development, as the results are bound to be a trifle hard and " contrasty." Incandescent gaslight is better for portraiture, as a full-size mantle gives an illumination of about sixty candle-power when at its best, and is about one and a-half times as effective photographically as a gas flame of the same visual intensity, because of the whiteness of the light and its richness in the blue or actinic rays. The exposure depends, of course, upon the num ber of burners. The special fittings obtainable from factors are in the form of brackets containing a score or more lights, which make possible quite brief exposures—say one or two seconds ; but an enormous amount of heat is produced. Pair
results may be obtained with one or more ordinary domestic burners, but the exposure is somewhat long, a minute at least, with two lights near to the sitter, a very rapid plate, and the lens at f/8.
It will be noted that all gaslights are in candescent, the light in a plain flame being produced by heating particles of carbon to in candescence ; but it is convenient to adopt here the ordinary nomenclature by which the term " incandescent " is restricted to burners of the Bunsen type, fitted with mantles of rare earths.
Acetylene outfits for portraiture are similar to the incandescent gas fittings in arrange ment. The light is very actinic, and as a rough guide it may be said that when using fourteen acetylene lights, a rapid plate, and a lens at f/8, an exposure of about five seconds is required in an ordinary room ; the distance of the lights from the sitter greatly affects the length of the exposure. (See also " Artificial Light, Photo graphy by.")