PHOTOGRAPHIC MATERIALS 121 



unless special precautions are taken, the safe processing temperature is at least 20°F. 

 below the melting point in water. This would indicate that hardening might be 

 desirable with all emulsions, but since the hardening reduces the swelling and thus the 

 penetration of processing solutions, it may affect the developing characteristics 

 unfavorably; consequently a high degree of hardening is given only where the con- 

 ditions of use demand it, as with emulsions for use in tropical climates. 



The melting point alone, however, is not the only measure of permissible processing 

 temperatures, since thickly coated emulsions are more susceptible to mechanical 

 damage when swollen than are thinly coated emulsions with similar melting points. 



Optical Properties. — Since the silver grains reflect a certain amount of the incident 

 light, the emulsion forms a turbid medium, in which the light is scattered to a certain 

 extent around the point at which it enters the emulsion. This causes a small spreading 

 of the developable image around the edges of the optical image; this spreading is 

 called "irradiation." The amount of spread is proportional to the exposure, in any 

 particular emulsion, but for most work the effect on the definition is not of importance 

 except for very great exposures. When the negative image is to undergo considerable 

 enlargement in printing, as in miniature-camera negatives, overexposure should be 

 avoided to prevent loss of definition from irradiation. Also when precise measure- 

 ments are to be made on the developed image, this spreading must be taken into 

 account. The spreading is occasionally of practical value, as in the measurement of 

 brightness in astronomy. Since a star image is practically a point, it is too small for a 

 measurement of density. However, since the distance to which the irradiation is 

 effective is proportional to the brightness, the diameter of the developed image gives 

 a measure of the brightness of the star.^ 



Effects of Processing. — As has been mentioned, the gelatin swells to different extents 

 in the various processing solutions, but so long as a critical temperature is not exceeded, 

 it shrinks on drying to substantially its original form. The presence of a silver image 

 does cause minute changes, however, so that, for instance, two adjacent point images 

 tend to draw closer together during the processing, and, in extremely precise measure- 

 ments of position, this factor must be considered. Also, since near the edges of a 

 plate the strains are not evenly balanced, a slight distortion may occur. For this 

 reason precise measurements are never attempted near the edges of a plate. 



Supports. — The material used as a mechanical support for the emulsion must first 

 of all be photographically inert, i.e., it must have no deleterious action on the emulsion 

 before exposure, on the latent image produced by the exposure, or on the final silver 

 image produced by development, and it should not be affected by the solutions used in 

 processing the exposed emulsion. The physical properties required vary considerably 

 and are determined by the use to which the material will be put. Of the various niate- 

 rials which might be suitable, the only ones in common use are glass, cellulose ester 

 films, and paper. 



Glass. — For many years glass was the chief, and is still an important, base for nega- 

 tive emulsions and positive transparencies. It is practically completely inert and 

 transparent, and it maintains its form without bending and without any shrinking or 

 swelling during or after processing. However, it has the disadvantages of weight, 

 bulk, fragility, and, for some applications, nonflexibility. Therefore it has been 

 superseded for most purposes by the more convenient cellulosic films, though it is still 

 used for those special applications where rigidity and absolute freedom from swelling 

 or shrinking are important. Also, because of the possibility of coating single plates by 

 hand or small batches of plates by machine, it is generally used for experimental 

 emulsions which are used only in small quantities, such as the various specially sensi- 

 tized emulsions used in spectroscopy and astronomy. 



1 R088, F. E., "The Physics of the Developed Photographic Image," Van Nostrand (1924). 



