OUTLINE OF PHOTOGRAPHY 3 



have been available. Color photography for the amateur is gradually approaching, 

 for already it is possible for him to purchase color film and make his own trans- 

 parencies. Making prints in color from this color film or from color-separation nega- 

 tives is still such a complicated and costly process that relatively few amateurs 

 have attempted this field. But, in time, color processes will undoubtedly be developed 

 into such a form that the average amateur can make complete use of color. Black- 

 and-white prints may then be expected to be seen only in exceptional cases or where 

 the utmost economy must be effected. 



In practically every phase of industry, photography is employed at the present 

 time. It is impossible to enumerate the various uses to which photography may be 

 put, but perhaps enough has already been written to indicate its importance. What 

 has been accomplished in the past provides a background by which we may make some 

 type of estimate of the developments yet to come. 



Elementary Photographic System in Monochrome. — All modern photographic 

 processes depend for their operation upon the photochemical action which takes 

 place when "light" strikes certain sensitized materials. In the vast majority of cases 

 which are referred to as photographic processes, the photosensitized material consists 

 of a thin layer or emulsion of a silver halide deposited on a transparent supporting 

 base of glass, nitrocellulose gelatin, or paper. A photochemical change occurs in the 

 silver halide grains which are exposed to light, although the exact nature of this 

 alteration has not yet been entirely satisfactorilj^ explained. Exposure of the grains 

 of the silver halide produces a "latent image" of the object focused on the photo- 

 graphic material by a lens system. With the proper chemical processing operations, 

 the exposed grains of silver salt are changed to metallic silver, and the unexposed 

 grains are dissolved out of the emulsion. Since the silver grains are black when 

 viewed by transmitted light, the light portions of the original image become dark, 

 and the dark portions of the original object become more or less transparent. Thus 

 the light and dark portions of the original object have been reversed, and for this 

 reason the combination of the developed silver grains on their support of glass or 

 gelatin is known as a negative. 



If we now use this negative, in conjunction with a light source, as a new image 

 and go through another similar photographic process, we can obtain on a glass plate, 

 gelatin film, or, more usually, a paper base, another image. However, this second 

 image will be reversed in intensity from that of the first negative; it will be a negative 

 of a negative, and will therefore show bright and dark areas in monochrome in accord- 

 ance with the original image. 



This is the essence of the more common black-and-white photography. The 

 details of the various operations will be dealt with in subsequent chapters of this 

 book, but a brief introduction of the essential operations will be given in this chapter 

 to provide a certain degree of unity and coherence which could not otherwise be 

 obtained in a reference volume such as this handbook. 



Ldght Source. — To actuate the photographic plate, the light source which is to be 

 used must produce some radiation in the spectral region in which the sensitized plate 

 is sensitive. All usual photographic materials are most sensitive in the blue end of 

 the visible spectrum. Those photographic materials which are sensitive only to the 

 blue end of the visible spectrum are usually known as "ordinary" or orthonon mate- 

 rials, indicating that their sensitiveness does not extend through the complete visual 

 spectrum. Orthochromatic materials are sensitive in the blue, green, and, more or 

 less, the yellow portions but are insensitive to the orange and red regions of the visible 

 spectrum. Panchromatic materials are sensitive to all portions of the visible spec- 

 trum but still are more sensitive to the blue than red. Therefore, so far as the light 

 source is concerned, almost any source of visible light can be used with panchromatic 



