The advance of photography : its history and modern applications (1911)

THE CHEMICAL ACTION OF LIGHT 53 chemical action of the rays of light must be more intense, and this has been proved by experiments on the Alps to be the case. But not only are chemical effects produced by direct sunlight ; the light of the blue sky, which is nothing but reflected sunlight, is likewise chemically active, and powerfully so, through its blue colour. It has been already stated that the blue colour of the sky proceeds from the scattering of blue light which takes place in the passage of sunlight through the air. The quantity of this scattered light varies with the hour of the day, being strongest when the sun is highest (that is, at noon), and diminishes in proportion as the sun approaches the horizon. Photographers therefore choose the middle of the day — i.e. between 10 a.m. and 2 p.m. — for taking portraits, for which the light of the blue sky is preferred. During these hours the chemical effect of this light remains almost the same ; afterwards it begins to diminish — quickly in winter, more slowly in summer. Thus the chemical power of light, according to Bunsen, expressed in degrees, is at Berlin : — 12 o'clock. 1 o'clock. 2 o'clock. 3 o'clock. 4 o'clock. 5 o'clock. 6 o'clock. 7 o'clock. 8 o'clock. June 21 . 38° 38 38 37 35 30 24 14 6 Dec. 21 . 20° 18 15 9 0 0 It appears from this example how extraordinarily weak is the chemical action in winter (thus about noon on the 21st December it is only half as powerful as at noon on the 21st June) ; moreover, how small is the amount of chemical light which is diffused by the blue sky on the 21st December, on account of the shortness of the day. Therefore a longer exposure is necessary in winter than in summer, and, the printing process being slower, a longer time is required in winter to print the same number of pictures. Now, the intensity of the blue sky light depends on the position of the sun, and the latter varies, not only