As a simple proof of the correctness of this result, a piece of slow tinting medium held within the spot of light near the surface of the cloth reveals a first appearance time of 2 seconds, which time has already been found to indicate 32 actinos of intensity for a blue surface, or 64 actinos for a white one (p. 62).
As a further illustration suppose a common lamp flame approximately 1 inch in diameter, illuminates a fair complexioned face at 8 inches distance; what is the actinicity of the face? It has already been found (p. 54) that the average actinicity of a kerosene flame is 32 ac tinos and therefore according to the first rule : x 64 (cone units) x 32 x (actinic factor of a fair complexion) = A, of an actino, the intensity of the face. Or by the second rule, 4 x 32 ÷ = a and x = A of an actino, the intensity of the face as before.
To prove this result the incident light time is taken at 1/ 2, at 2 inches from the flame (with the standard tinting medium, to save time), and the tint is secured in 8 seconds. It would be 16 times this time or 128 seconds at f/ 8, the flame's convergence at the head, and 8 times this time or 1M seconds with the slow factor 8 tint ing medium used in these calculations. Now
since a 64 second first appearance time on this factor 8 medium indicates one actino of inten sity for a fair complexion, a time of 1M seconds indicates an intensity of A of an actino as before.
As an example of exposure and to illustrate the practicability of even small and compara tively weak artificial lights for portraiture suppose that this head be photographed on a plate whose speed is 64 seconds or one minute and with a lens working at 256 cone units, (f / 4). On dividing 64, the speed of the plate, by A, the actinicity of the face, and this result by 256, the unit value of the full lens opening, a result of 4 is obtained, the time of exposure in seconds under these conditions. A bank of 8 gas light flames, working at 16 diameters of the flame from the subject would probably create about four times the above intensity and would be very practical for bust portraiture, if the most rapid plates and lenses were used.