548 HANDBOOK OF PHOTOGRAPHY 



2. The original silver deposit may be treated with substances which will unite with 

 the metallic silver so as to produce a combined deposit of greater density than that 

 provided by the metallic silver grains alone. Chromium, mercury, lead, copper, and 

 other metals may be used to combine with the original silver. While some of these 

 methods produce an appreciable increase in density, the intensified image may not be 

 neutral in tone. 



3. The color of the original deposit may be altered to provide greater opacity to 

 the passage of light through the negative. In such cases the visual and photographic 

 intensification may be quite different, so that visual inspection does not give a good 

 indication of the photographic effect which this type of intensification produces. 

 This is especially true where the color of the intensified negative is yellow, for a 

 yellow stain has relatively high absorption in the blue end of the spectrum to which 

 printing materials are most sensitive. 



Instead of classifying the intensification process, as we have done, according 

 to its modus operandi, a classification based on the visual and photographic effects 

 is sometimes employed. According to this classification there are three types of 

 intensification : 



1. Intensification methods giving both visual and photographic intensification. 

 This is the most common and useful class of intensifiers. 



2. Methods giving photographic intensification but visual reduction. 



3. Methods giving visual intensification with photographic reduction obtained 

 only when intensifiers having a bleaching effect are employed with negatives which 

 were originally considerably colored or stained. 



Considerations in Intensification. — The characteristics of intensifiers as they 

 affect the original negative should be considered in selecting an intensifier for any 

 particular purpose. Permanence of the intensified image is important when prints 

 are to be made over a long period of time from the same negative. Another factor of 

 considerable importance, especially in scientific work, is preservation of gradation of 

 the original image. 



Since the desired degree of intensification may not be obtained with a single 

 treatment, it is important to determine whether or not the particular method chosen 

 permits repeated intensification by successive treatments. The color of the intensified 

 image may also have important practical consequences, for it may be extremely 

 difficult to determine by visual inspection the extent of photographic intensification 

 when the intensified image is colored. Thus colored images which result from the use 

 of such intensifiers as uranium and the copper-tin mercuric iodide intensifier followed 

 by Schlippe's salt or an alkaline developer may produce considerably greater photo- 

 graphic intensification than might be judged from visual inspection. 



Excessive intensification may have been attained in the intensification process. 

 In such cases it is advantageous to know the conditions under which the intensified 

 image may be reduced. 



The important characteristics of the more useful intensifiers are given in Table I. 

 For details on other intensifiers, or for additional data not given here, the reader is 

 referred to more complete articles given in the bibliography. 



Sensitometry of Intensification, — The quantitative measurement of the inten- 

 sification which is possible through the use of various intensifjdng agents may be 

 determined and expressed in much the same manner as the sensitometric character- 

 istics of photographic materials.^ 



In determining the characteristics of intensifiers, a strip of film is given a series of 

 known, graded or stepped exposures and is then developed, washed, fixed, and dried in 



' Suitable methods for determining sensitometric characteristics have been given in the chapter on 

 Photographic Sensitometry. 



