It may be well here to call attention to a truth of nature which concerns the study of it by this method and that is that the expression "the actinicity of a surface" always implies the actin icity as effective at the point of the impinging cone where a measurement is taken. Just as we see objects in nature through a layer of air, smoke or mist, so also the lens must photograph them. It is in this sense that the sun may be said to have an actinicity of 32MM actinos at the zenith, one half of that at from the hor izon and, as the author has found under clear weather conditions only 64 actinos when its lower rim rests on the horizon.
Problem 2: To Find the Actinicity of a Diitant Surface of Small Convergence and Low Value. The moon is a good example under this problem. The actinicity of such surfaces may be found by securing the least visible tint in the position of the image of such a surface, that image being formed by a lens working at a known convergence. The f value of the lens as it casts the image is taken as the basis of cal culation instead of the f value obtained as in the problems of the sun and of the kerosene flame.
For example, on a clear night when the moon is 30° from the horizon its image is cast by a lens working at 1/ 4 and on taking the time with the standard film in the position of that image it is found to be 32 seconds. The problem from now on is exactly as the previous one of the kerosene flame. This time 32 seconds divided by 16 (42) equals 2, the time in seconds at f / 1 and 64 divided by this number gives the actinicity of the moon's surface at that altitude as 32 actinos.
The use of the lens is simply to increase optically the cone value of the rays impinging from the source measured and thus to reduce the tint time to a more practicable length for measurement. The opacity of the glass in the lens retards to a measurable degree the strength of the light, but probably not over 20 per cent which is a negligible amount of it and the re sult, with this exception, is the same with the f/4 lens as though it were possible to approach the moon itself to a point four times its diameter from it.
Problem 3: To Find the Actinicity of Any Surface by Taking the First Appearance Time at That Surface or, in Other Words, of the Light Incident Upon it. On measuring the light emitted from any surface, as already explained, the standard f / 1 meter is used and should 64 seconds be required under any conditions to create the standard or least visible tint on the standard medium, it is known that the surface has an intensity of one actino. Should this tint
be secured in 1 / 64 of that time or in 1 second, then it would be known that the actinicity of the surface equals 64 divided by 1, or 64 actinos. This standard method should be well under stood by the reader by this time and is repeated here only for purposes of comparison. In ob taining the actinicity of a surface by the measure ment of the light incident upon it, allowance must be made for the difference in what may be termed the inherent actinicity of surfaces by reason of their different colors or shades. This is that inherent quality that makes blue more intense than green and white more actinic than gray when in equal light conditions.
Suppose for example that a white, a blue, a yellow and a black cloth be arranged in the same light and that their respective actinicities be measured with the standard meter. The white cloth will be found to be the most actinic and each of the others to measure less than the white in the order named. Should however the light at these surfaces or incident upon them be measured instead of that emitted from them, it is evident that the color of the surfaces could have no effect on the measurement, since exactly the same light falls upon each surface and the tinting medium would be turned di rectly toward that light instead of toward the surfaces.
Considering therefore the incident light, it is evident that one which would suffice to bring a white surface to a one actino intensity would not create so great an intensity in black, since it may be shown that ordinary black, as of cloth texture, has only approximately 1 /16 the actin icity of white in the same light. This being true a black cloth would require to be illuminated with 16 times as strong an incident light as the white one in order to create the same actinicity.
To find therefore the actinicity of any partic ular surface by measuring the light incident upon it, it is necessary only to find what would be the first appearance time of incident light of such intensity as to bring the surface to one actino.