The sun's diameters The p.o.p. time of the above the horizon light 1 or less 8 seconds 1 to 2 4 41 2 to 5 2 " 5 to 12 1 " From this point upward the zone may best be ascertained by standing on level ground and noting the length of one's shadow as compared to his height. This ratio may of course be found by comparing the length of any shadow on a horizontal plane, with the height of the object casting that shadow. For example on a card, held horizontally, may be placed the back of a knife blade, the edge extending upward, and if the sun be shining across the blade at right angles to it the relation of the shadow length to the height of the blade may easily be observed. These ratios are now given which indicate the p.o.p. times of one second and less.
Length of shadow compared to p.o.p. time of the height of object casting it light 7 to 14 times 1 second 4 to 7 " 1/2 " 1.5 to 4 " 1/4 " less than 1.5 times 1/8 " Should one be acquainted, however, with the disappearing scale of tints (p. 41) even the brightest sunlight may easily be measured on p.o.p. with a "contact opening" and although it should never be necessary to do so, it is well enough to know how such a measurement may be made. When the day is clear and the sun in the maximum zone, 40 or more degrees from the horizon, a quarter second exposure of a slow tinting medium will give a depth of tint which immediately should be recognized as the second visible tint of the disappearing scale. Without actually making an eighth second ex posure, which is too fast to give with the unaided hand, it is evident then that such an exposure would give the least visible tint and that the p.o.p. time is really i second. Of course, when the sun is nearer the horizon the light can easily be measured and it is recommended to do so.
be able to estimate the intensity of sun light at a glance, however, through a knowledge of the intensity zones, is a useful and desirable accomplishment since it saves one the trouble of measuring bright sunlight and also impresses the worker with the stable character of that kind of light. On dropping below the horizon the light becomes weaker more or less rapidly according to the angle at which the sun has passed it. In the tropics after sunset night approaches very quickly indeed but in the lati tude of Norway, for example, when the season is such that the sun just drops below the hori zon at midnight the p.o.p. time of the light in clear weather may be 16 or 32 seconds at that hour, since midnight in such a latitude may compare in brightness with early twilight in the temperate zones.
Under all such conditions of weak light it has already been explained that in order to save time a faster tinting medium may be used and the time as found multiplied by the factor of the slow p.o.p. as compared to it in order to secure the time as though taken with the p.o.p. It is frequently convenient to use one of the so called "gas light" papers, such as Argo, Cyco, Velox, etc., as these papers on the average require only about :the time to tint as is required by the gelatin papers. By taking the p.o.p. time of the light with both gelatin paper and any convenient gas light paper at two or three steps from a window it may easily be seen how many times faster the latter is than the former and under weak light conditions the time found with the faster paper, multiplied by this number or factor will give the p.o.p. time as though meas ured with the gelatin paper. (See pp. 41, 47.)