ACTINISM AND DIAPHRAGM SYSTEMS In order to apply rudimentary knowledge to the practice of photography it is necessary to develop in the mind the idea of a value not be fore met with — that of actinicity. This is de fined as the capability of radiation to produce certain chemical changes on converging at a point. This property is possessed by light from all surfaces about us ranging from that of the sun to that of some surfaces quite invisible. Until it is fully understood that each surface in nature and each subject as it comes before the lens has in its different planes actinicity to a measurable degree and that these values are expressible in simple numbers, the problems of photographic exposure cannot be intelligently solved.
It will be shown that the speed of a plate or film may be for example one minute (or 64 seconds which may be substituted for one minute as it is one of the terms in the geometric series of base 2 as used throughout this treatise; pp. 21, 26), and this time is simply the proper or normal exposure with that plate or film when there exists in the subject a one unit actinicity and the unit diaphragm is in the lens. Now if it be ascertained by measurement that the actinicity of a surface is 16 units, then it is evi dent that the required exposure will be only of that which would be required should the sub ject be of only one unit actinicity, or in the above supposed case, A of 64 seconds or 4 seconds. Again, since this 4 second exposure is calculated on the basis of the unit diaphragm it is evident that should the 4 unit diaphragm be used the exposure would be only 4 of 4 seconds or 1 second. That is to say, the speed of the emul
sion divided by the actinicity of the subject gives the exposure with the one unit diaphragm and this exposure divided by the diaphragm used gives the exposure with that diaphragm.
This simple measurement, which is so easily and quickly made in any case, should end for ever those unnecessary complications due to the hour of the day, the season of the year and the latitude in which one is working, as well as those due to temporary conditions of the weather and the color or inherent actinicity of the sub ject to be photographed (p. 59).
It is a simple enough idea to which we are educated, to think of the temperature of a room as being for example, and we do this without any special or technical knowledge of the science of heat. Also then without being highly edu cated in physical optics the reader is asked to recognize the special capacity of radiation to effect chemical changes as in photography and which is called actinism. Light from all sources is actinic to some degree, and we speak of the actinicity of a source, of the sky for example, as a measure of the quantity of effect produced under specified conditions by the light converg ing from this source. The appreciation of the reality of this property and the ability to measure it with certainty or with all the exactness re quired, go a long way toward establishing a firm scientific basis for the practice of photog raphy.