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THE CHEMICAL ACTION OF LIGHT 51
with the more bulky apparatus which is commonly used for this work.
The flash bag is shaped as in diagram. The loop at the end of the thread can be attached to a nail or some suitable support, the end of the fuse ignited, and then sufficient time elapses before the flash takes place to enable the operator to move quite out of the way, or even to attach himself to the group if he so desires.
As more continuous forms of light having intense photographic effect are often required, use is made of the lime-light, incandescent gas, and electric lights for this purpose.
The two latter, together with acetylene lamps, are particularly useful for working with special classes of prepared papers, which are now being so largely used in the place of P.O. P. and also for copying and enlarging work.
The employment for portraiture of such dazzling artificial light as the electric light is attended with the great drawback of occasioning harshly defined shadows, which do not add to the beauty of the portrait. This difficulty is sometimes modified by placing another electric light of less power on the shaded side, but under such circumstances it is difficult, as in direct sunlight, to prevent contraction of the features.
Photographic Action of Sunlight. — Sunlight is the most important source of light for photographic purposes. The brightness of this light is, however, subject to great variations. Even the eye recognises that the sun is much brighter at noon than in the morning and evening. According to the measurements of Bouguer, this difference is so considerable that the sun at an elevation of 50° above the horizon is 1200 times brighter than at sunrise. The eye, moreover, perceives a decided difference of colour between the sun on the horizon and the sun at the zenith. The latter appears white, the former of a reddish hue ; and, on examination with the spectroscope, it is found that in the