Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER H ACTINISM AND DIAPHRAGM SYSTEMS A NEW PHYSICAL QUANTITY WE are endowed by nature with what may be termed a sense of quantity values and it is this sense which is gratified by the use of measurements of all kinds. 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 before 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 64seconds 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 evident that the required exposure will be only /6 of that which would be required should the subject be of only one unit actinicity, or in the above supposed case, 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 on...