PHOTOGRAPHIC MATERIALS 147 



halide can be blackened, by exposure and development or bj^ chemical action, to give 

 the desired positive. 



In ordinary emulsions, there is much more silver bromide than is used in a normal 

 negative image, and the highlights of the residual positive will be dense and fogged. 

 Therefore emulsions intended for reversal processing are thinly coated so that the 

 high lights of a normally exposed, negative image will use up practically all the silver 

 halide. 



There are always some very small insensitive grains which will not be affected by 

 the first development, even in heavily exposed areas, and thus remain to fog the 

 high lights of the positive. Therefore a silver halide solvent, such as ammonia or 

 hypo, is often used in the developer or in a rinse bath after development, to dissolve 

 out these small grains, thereby increasing the brilliance of the positive image. 



When all the residual silver halide is utilized in the positive, there is no latitude for 

 the original exposure, and any small local variations in thickness, which in ordinary 

 work would be of no importance, have a marked effect. While some compensation 

 for variations in negatiA^e exposure can be made by regulating the action of the silver 

 halide solvent, the range of control is limited. A much greater degree of control can 

 be exercised by taking advantage of the differential sensitivity of the residual silver 

 halide grains and giving a controlled second exposure. Thus, where the negative 

 exposure was heavy, there will be little silver halide left, and it is given a heavy 

 exposure to render it all developable. On the other hand, where the original exposure 

 was light, there will be a great deal of silver halide left and a slight second exposure is 

 given, so that only part is rendered developable and the final positive is not too 

 dense. 



In this process, after removal of the negative image, the film is scanned by a photo- 

 electric cell which controls the intensity of the second exposure. Where the negative 

 image was heavy, there is less residual silver bromide, so that more radiation reaches 

 the photocell, which in turn increases the second exposure. With a thin or under- 

 exposed negative image, the reverse occurs. Thus the density of the positive image is 

 compensated as described above, and positives of good quality are obtained with a 

 wide range of camera exposures. 



Reversal Materials. — Screen-plate color materials, such as the Lumiere Autochrome 

 plates and Dufaycolor films must be processed by reversal so that the positive image 

 will be in register with the screen-filter elements. They are usually processed with an 

 ammonia first developer and a strong second developer which completely blackens 

 the residual positive image. There is little or no latitude in exposure. 



Substandard motion-picture films, particularly for amateur use where only one 

 positive is ordinarily needed, are processed by reversal in order to obtain better defini- 

 tion and lower graininess and to reduce the cost. Since exposures are made under 

 widely varying light conditions, some type of compensating processing is practically 

 a necessity. The Kodak automatically controlled second exposure was devised to 

 compensate for varying exposures throughout a single length of film. 



"Direct positive" paper for automatic portrait machines has a fairly fast fine- 

 grain orthochromatic emulsion thinly coated on a waterproofed paper base and is 

 processed by reversal in order to reduce the time between exposure and deliverj^ of the 

 finished print. Lighting and exposure are standardized, and the whole residual posi- 

 tive is darkened either by full exposure and redevelopment or, more generally, bj'^ 

 conversion to silver sulphide. 



In addition to the regular reversal process, there are other methods of securing the 

 effect of a positive image directly. The so-called "tintype" made use of a collodion 

 emulsion coated on a black support and developed to give a whitish silver deposit, 

 which then appeared as a positive against the dark background. 



