THE METHOD AS PRACTISED The foregoing method of reducing the actin icity and diaphragm to unity has been given to make clear in a simple manner the theory involved in measuring the speed of an emul sion. In "speeding an emulsion" in practice, however, it is unnecessary either to alter the strength of light on the subject or to close down the diaphragm to unity.
Suppose that it be desired to make test ex posures say of 8, 4, 2 and 1 minute duration under unit conditions of actinicity and diaphragm. All that is necessary is to observe that the ex posures given to a surface of any actinicity and with any diaphragm in the lens are equivalent to those unit condition exposures which it is.
desired to give. For example should the light on the sheet when first measured in the full light of the window have measured 8 actinos, then the series of unit test exposures of 8, 4, 2 and 1 minutes which it is desired to give under unit conditions are reduced to their equivalent exposures, still with diaphragm one of the unit scale in the lens, by dividing each of the series by 8, the actinicity of the surface in actinos. The series becomes then the intervals of 1, ;, and A minutes, or better — 64, 32, 16 and 8 seconds. Should it be desired to shorten the exposures still more they may be divided by the number of any diaphragm used. For ex ample if the 4 unit instead of the 1 unit dia phragm be used these exposures would be reduced to one fourth of those last given or to 16, 8, 4, and 2 seconds respectively.
A very convenient method is to divide 64 by the actinicity of the surface photographed, when the quotient will be the number of the unit scale diaphragm, which if used will allow the test exposures in minutes to be given in seconds. For example suppose that the surface used in the test measure 16 actinos; dividing 64 by 16 is obtained 4 and should the 4 unit diaphragm be used (f/ 32) the test exposures in minutes may be reduced to the same number of seconds, in this case the test exposures of 8, 4, 2 and 1 minutes being reduced to 8, 4, 2 and 1 seconds. This is for the reason that the combined condi tions are 64 times as fast as unit conditions, the 16 actinos of the surface photographed being 16 times as fast working as 1 actino and the 4 unit diaphragm 4 times as fast as the 1 unit diaphragm; the total greater rapidity is there fore 16 times 4 or 64. Therefore for each 64 seconds or 1 minute of the desired unit test exposures there will suffice 1 second under the faster conditions. Should the intensity be 8 actinos and the diaphragm 8 units, or the actinos 32 and the diaphragm 2, the result would of course be the same.
The reader should understand fully that every normal exposure (once the speed of an emulsion is found) is equal to the speed time exposure of that emulsion under unit conditions of light and diaphragm. For example should it be found that the speed of an emulsion be 8 minutes, every normal exposure is equal to 8 minutes exposure with unit intensity and unit diaphragm. This has already been explained and will be made still clearer by the two following ex amples.
Suppose that a plate having a speed of two minutes be used to make a short exposure of a running horse of average color; that the day is clear and the sun high and that another similar plate be used to make a long exposure of the interior of a room. It will be shown that these exposures when reduced to their equivalents with unit intensity and unit diaphragm con ditions, are exactly equal chemically. In the table of one unit actinicities to be used in prac tice (p. 143) it will be seen that an average view is considered as having 64 actinos of intensity when the p.o.p. time of the incident light is one second and since under the conditions men tioned the time would be i of a second, the actinicity of the subject must be 8 times 64 or 512 actinos. The exposure with the 1 unit dia phragm would therefore be 5=a of 128 seconds, the speed of the plate, or i second. Now should diaphragm 256 unit scale, or f/4, be used, the exposure will be -:,. of ,I, second or m% of a second. Therefore an exposure of ., of a second to a surface having 512 actinos of intensity and with a 256 unit cone in the lens is equivalent to an exposure of 128 seconds with unit conditions of intensity and diaphragm.
The other example as already stated, is that of a room interior which is given 8 seconds with the 8 unit diaphragm (f/ 22), the brightest part of the wall having measured 2 actinos. Had the wall measured only one actino the exposure with the same stop would have been 16 seconds in stead of 8, and had the one unit diaphragm been used instead of the 8 unit one the exposure would have been 8 times 16 seconds or 128 seconds which, like the previous fast exposure, is seen to be equivalent to the speed time exposure with unit conditions. It is evident therefore that the two plates in the examples given suffered approximately the same chemical ex posure.
Any normal exposure multiplied by the unit number of the diaphragm used and then by the actinicity of the subject in actinos gives the speed-time of the emulsion used. It must also be true therefore that the speed-time divided by any two of the three factors of diaphragm, actinicity and exposure, will give the remaining factor.
Algebraically the relation is simply expressed T i by the equation t= which t is the time of exposure in any case, T the speed-time of the plate used, a the aperture in unit cone value, and I the actinicity of the light source.
Should the worker prefer he may proceed with the speeding tests on the basis of the author's suggestions regarding speed in the back of the book. The foregoing experiments are recommended however since by performing them one cannot but arrive at a full understanding of the truths involved. In regard to the speeds as suggested, should the exposures after trial seem in the judgment of the worker to be rather under normal the speed may be considered as twice the time used in the experiment. On the con trary, should they seem to be over-exposed then the speed-time may be considered as one half of that tried.