TECHNIQUE OF DEVELOPMENT 373 



Rapid Processing. — All the formulas considered so far for materials other than 

 papers have been designed for developing times of a number of minutes. In some 

 cases, it is very desirable to develop the emulsions in much less time and various 

 formulas have been worked out to reduce developing times to a minimum. Needless 

 to say, such rapid processing methods can rarely be worked out to give the extreme 

 activity with as good quality and control as the more usual formulas. Use is made of 

 accelerating action of high alkalinity and concentration, so these formulas are not 

 particularly economical, but they find a place where rapidity of development is of 

 paramount importance. 



One formula is given below: 



No. 23 Rapid Developer 



Sodium sulphite (anhydrous) 187.0 g. 1 lb. 9 oz. 



Rhodol (metol or Elon) 21.5 g. 2 oz. 375 gr. 



Hydroquinone 43.0 g. 5 oz. 315 gr. 



Sodium hydroxide 43.0 g. 5 oz. 315 gr. 



Water to 1 1. 1 gal. 



The developing time is usually of the order of II2 min. at 65°F. After develop- 

 ment a 5 per cent acetic acid stop bath should be used, followed bj^ fixation in hypo 

 with good hardening action. 



Physical Development. — The primarj' distinction between chemical and physical 

 development is the source of the silver. In chemical development the silver deposit 

 is essentially produced by reduction in situ, of the original halide. In physical 

 development the silver is originally in the developer itself and deposits out on nucleuses 

 to form the final image. The distinction becomes somewhat arbitrary and hard to 

 define for a limiting case such that the first action of the developer is to dissolve silver 

 halide, thus charging the developer with a soluble silver salt, then subsequent deposit 

 from the solution on the nuclei. Probably ordinary development partakes at least 

 to some extent of this nature, though the action of depositing silver is probably very 

 local in nature and the body of the solution never contains any great quantity of 

 silver salts. 



Physical development is of interest for two reasons, as it gives an additional tool 

 for the study of the nature of the latent image and as it produces images of unusually 

 fine grain under the best conditions. 



Physical development can be carried out after fixation, thus proving that the 

 "latent image" is in reality of quite different nature than the silver halide itself. 

 Fixation in a neutral or alkaline hypo solution is more favorable for this effect than 

 the use of an acid fixing bath. This may be connected with the fact that acid fixing 

 baths tend to attack or dissolve the silver of the developed image. Possibly there is a 

 similar solvent action on the minute quantities of whatever material composes the 

 "latent image." The probability of this connection is enhanced by the fact that there 

 is other evidence which also indicates that the latent image is silver deposited in 

 submicroscopic quantities in the gelatin. 



In handling physical developers difficulty is often experienced, as they tend to 

 deposit silver on other surfaces, such as the sides of the dish in which the development 

 is taking place. The developing formulas are closely related to baths such as are 

 used in silvering mirrors. The difficulties of the process and the very low effective 

 emulsion speeds obtained, coupled with the small advantages to be gained, have kept 

 physical development in a minor role in photographic processing. 



Some attempts have been made to secure the fine grain rather generally credited to 

 physical development without all its attendant disadvantages and uncertainties. 

 Some success has been reported, but some others have failed in their attempts to 

 obtain similar favorable results. The reasoris for these diverse and opposing views 



