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

54 THE ADVANCE OF PHOTOGRAPHY according to the different seasons, but also at the very same seasons on different parts of the earth. If circles be drawn round the earth from pole to pole, we obtain what are called meridians (ra ra, fig. 18). At all places situated on the same meridian it is noon at the same time, but the height of the sun varies very much according to the distance of the place from the equator. If circles be drawn round the earth parallel to the equator, they form the so-called lines of latitude. If the sun is perpendicular at noon at a particular place on the equator, at 10° of north latitude it is 10° lower — that is, the height of the sun (or the distance of the sun from the horizon expressed in angular measurement) is 80°. At 10° further north, the position of the sun, at the same time, is only 70° ; and at the pole, which is 90° from the equator, the height of the sun=0 — that is, the sun is on the horizon. lg' ' The chemical action of the blue sky light varies greatly, corresponding to the different positions of the sun at the same time. Thus, for example — At Cairo, on the 21st Sept., the strength of light at noon = 105° At Heidelberg „ „ „ = 57° In Iceland „ „ „ = 27° Therefore, the lower the latitude of a place is, the richer it is in the amount of light available to the photographer. Accordingly, the American photographers are better off in this respect than those of Germany and England. These differences in the chemical intensity of light are also essentially modified by the state of the weather. If the sky is covered with grey clouds, the chemical intensity of the light is considerably less than with a perfectly clear sky. On the other hand, white clouds increase the power of the light very decidedly. In the autumn the