This time divided by the first appearance time at the surface as actually measured in any given case will give the actinicity of that surface in actinos.
This one actino time as referred to above will now be found for different surfaces and given in a table. In working out this table the slower, or factor 8, tinting mediums are used and are recommended to be used in general practice for measuring incident light, since as already explained, the standard medium is too sensitive to be observed with ease under such intense light conditions. Should the light be so weak however as to make it advisable to gain time in taking the measurement, as after sunset or in dark interiors, the standard medium may of course be used and its time multiplied by 8, to reduce it to the equivalent p.o.p. time, which is used as the basis of calculation in this problem.
As the four surfaces mentioned above are arranged in the same light, as explained, a standard measurement of the white surface, with the standard tinting medium and the fl1 meter, results in a time of one second and its actinicity is found therefore to be 64 actinos, since 64 divided by 1 equals 64. Now on measuring the incident light with the slower tinting medium as recommended in this prob lem, the time is found to be 2 seconds. A 2 second incident light time with the slow tinting medium indicates therefore a 64 actino intensity for a white surface and from this it is evident that had the incident light been twice as strong the incident light would have measured 1 second instead of 2 and the intensity of the white surface would have been 128 instead of 64 actinos. Also it is clear that if a one second incident-light first appearance time with a factor 8 tinting medium indicates 128 actinos of inten sity for a white surface, a 128 second incident fight time would indicate an intensity of 1 actino. The one actino, incident light first appearance time with a factor 8 tinting medium is found therefore to be 128 seconds for a white surface.
To illustrate suppose that under certain con ditions the first appearance time of the light incident upon a white surface, as measured with solio paper, is 8 seconds: Now it has been found and is a fact that a first appearance time of 128 seconds on such a tinting medium indicated an intensity of one actino in a white surface; therefore a first appearance time of seconds indicates an intensity of 16 actinos= 16 .
That is, a white cloth illuminated with such a light would measure 16 actinos.
Measuring now the blue cloth with the stand ard meter and medium its actinicity is found to be 32 instead of 64 actinos as for the white cloth. It is evident therefore that twice as strong an incident light would be required to bring it to any certain intensity, say to a one actino intensity, as would be required for the white cloth. From this it is seen that a 64 second first appearance, incident light, as meas ured with the factor 8 medium, is required to secure a one actino intensity for a blue surface instead of 128 seconds as required for the white surface. As yellow is about one half the actin icity of blue in the same light it is clear that the incident light must be double the intensity of that for blue, or 32 seconds.
Suppose furthermore that with p.o.p. held near a yellow cloth the incident light time is found to be 4 seconds. Now since it has been found that a 32 second incident light is required to bring a yellow surface to an intensity of 1 actino, a light 8 times stronger, or one of 4 seconds time will evidently create an intensity of 8 actinos for that surface. Again, since a one second incident time indicated a 32 acting intensity for yellow, a 4 second time will indicate 1 /4 that intensity or an intensity of 8 actinos. In like manner the intensity of a white cloth in that same light, is found to be 32 actinos.
To simplify this matter still further for the reader and at the risk of repeating, it may be said that under any condition, as for example in the shade of a house, should the incident light measure 1 second with one of the factor 8 mediums a white cloth would therefore be known to have 128 actinos, a blue one 64, a yellow one 32 and a red or a black one but 8 actinos. It is interesting to note in this con nection that since the maximum incident light from the high sun and sky never passes 1/8 second on a slow medium the maximum actin icity of these surfaces by daylight can only be eight times as great as that mentioned for a 1 second time, or for white 1M, for blue 612, for yellow 256 and for black 64 actinos.