_Diurnal and seasonal changes._ During the winter months on the Western Front photography from the air was only possible for two or three hours around noon, on clear days. This calls attention to another factor of prime importance, namely, the large variation in the intensity of daylight during the course of the day and during the course of the year. [Illustration: FIG. 101.—Variation of average daylight intensity during the day.] Measurements showing typical variations from morning to night are exhibited in Fig. 101, from which it appears that there is an increase in illumination of four to five times from 8 o'clock—when it would be considered full daylight for purely visual observation—until noon, while there is a corresponding decrease by four o'clock. Fig. 102 shows sets of measurements by two different authorities which give the average intensity of daylight for each month throughout the year. From December to July there is an increase of approximately ten times. From both sets of data it therefore appears that—neglecting the frequent occurrence of clouds which reduce the illumination to a half or a quarter or even less—a variation in illumination of forty or fifty times occurs between mid-day in summer and morning in winter. In the photography of stationary objects on the ground this range of intensities is easily taken care of by selection of lens stop and shutter speed. On the airplane it is quite otherwise, because the shutter speeds called for at the lower illuminations are much slower than the motion of the plane will allow. [Illustration: FIG. 102.—Variation of intensity of daylight through the year; two different sets of measurements.] =Haze.=—At low altitudes the brightness range is substantially that which would be obtained by photometric measurements of soil and vegetation made at the earth's surface. At higher altitudes, especially above 2000 meters, this brightness range is materially decreased by atmospheric haze. The significance of this lies in the fact that for safety from anti-aircraft guns, war-time aerial photography must be carried out at very great elevations. Toward the end of the Great War photographic missions traveling at from 5000 to 7000 meters were the rule. At these heights, even in very clear weather, a veil of bluish-white haze reduces the already small contrasts still more. Some means for overcoming the effect of this haze becomes imperative, therefore, in order to secure in the picture even the normal contrast of the object.