Photomicrography 41 



metallic silver, the image is fixed by dissolving out the more stable crys- 

 tals, on which the developer has not yet had time to act. 



The resultant image is thus blackened by the presence of silver in 

 those areas on which light fell and is rendered transparent by the re- 

 moval of silver bromide in those areas on which light did not fall. The 

 result, were there only a single layer of crystals, would be a black-and- 

 white image. In practice many layers of minute silver bromide crystals are 

 supported in a gelatin film. In regions on which very little light fell, only 

 the surface crystals are affected. The whole depth of the film, that is, all 

 the crystals in the film, are affected in areas exposed to intense illumina- 

 tion. A thin layer of minute silver crystals appears very pale gray; a 

 thick layer appears black. Hence the film is capable of recording a wide 

 range of tones. It cannot do so automatically. It can only do so if it is 

 exposed for just so long as will permit the faintest light to render the 

 surface unstable and the strongest light just to penetrate to the depths. 

 The actual intensity of the "faintest" and "strongest" is immaterial. The 

 total "exposure" must be adjusted so that the faintest may have just time, 

 and the strongest not too much time, to act. 



Development is similarly not an all-or-nothing process. All silver bro- 

 mide, exposed to reducing agents, will ultimately be reduced to silver. 

 The purpose of photographic development is ideally to reduce only those 

 crystals that light has rendered unstable; both time and temperature of 

 development must therefore be rigorously controlled. "Fixation," or the 

 removal of undeveloped silver bromide, is a less exacting process, since 

 metallic silver is only very slowly attacked by the silver-bromide solvents 

 employed. A final washing, to remove the fixing agent, leaves a relatively 

 stable image composed of silver in gelatin. 



The image so formed is, of course, a negative. That is, it is black where 

 there was light and transparent where there was darkness. It is reversed 

 to a positive, or print, simply by duplicating the process. The negative is 

 laid against, or an image of the negative is projected onto, another layer 

 of silver bromide in gelatin. This second layer is then, in its turn, devel- 

 oped and, provided that the exposure has been carefully controlled, will 

 reproduce in whites, grays, and blacks all the shades of the original image. 



There are, therefore, three steps in photography. First, the formation 

 of an image on a layer of silver bromide for the necessary length of 

 time. Second, the processing of this layer of silver bromide into a negative. 

 Third, the reproduction of this negative as a positive or print. The forma- 

 tion of an image through a microscope has been discussed in the last two 

 chapters. Let us, before dealing specifically with the projection of this 

 image onto negative material, discuss in more detail the types of this 

 material that are available. 



