354 HANDBOOK OF PHOTOGRAPHY 



Stagnant Development. — As pointed out before, stagnant development is a limit 

 rarely if ever attained, but in many practical cases agitation is so slight that it con- 

 tributes little to the final result. Many classes of photographic materials are fre- 

 quently developed by these methods. They include roll and cut films, radiographs, 

 plates, and motion-picture film when handled on racks. Only the simplest 

 mechanisms are required as the emulsion is merely supported, at rest, within the 

 bath. The virtue of simplicity is marred by the fact that small residual effects inter- 

 fere with the uniformity of the results. Unless the bath has just been thoroughly 

 stirred, there are apt to be small thermal currents moving the developer enough to 

 cause streaks. If the film or plate is put in quietly and smoothly, air bells are apt to be 

 formed and cling to the emulsion, resulting in round white spots in exposed areas. 

 Again, with such careful introduction of the film into the developer, clips and hangers 

 are apt to leave marks from swirls and eddies which they cause in the solution. The 

 introduction of a second film near one alreadj' partly developed may cause streaks on 

 that for similar reasons. Vibrations of developing trays or tanks caused by machinery 

 not associated with the processing at all may set up enough developer agitation to 

 produce streaks and uneven development, particularly near holes in hangers or near 

 clips holding films. 



The best practice, when this general type of development is to be used, includes: 



1. Thorough preliminary stirring of the developer to secure uniform conditions. 



2. Thorough agitation of the film or plate when first introduced to dislodge air bells. 



3. Avoidance of all conditions leading to local streaming or agitations of the 

 developer. 



Tray Developments. Hand Agitation. — Many systems and procedures have been 

 devised to secure agitated development in the shallow trays used for so much photo- 

 graphic work. They range from the simple turning or moving of prints to elaborate, 

 accurately timed rocking and brushing cycles. 



In the simpler schemes the agitation is secured by more or less random handling 

 of the films or prints. At times this is a definite program of moving the units from one 

 place in the tray to another. Sometimes this is combined with tray tipping to secure 

 greater agitation. With paper prints this is particularly practical, as the development 

 is usually carried out by inspection, and minor variation in the time taken to come to 

 the desired quality is not usually very serious. 



This simple system is not adequate where accurately reproducible results are 

 required. Accurate sensitometric testing necessitates the highest possible precision, 

 and hence great efforts at development control have been made in sensitometric labora- 

 tories. Three classes of systems have been introduced to meet the requirements, 

 differing in their mode of agitation. The three depend upon (1) rocked traj^s, (2) 

 brushes, and (3) squeegees and agitator blades. 



Rocked Trays. — In using rocked-tray development for sensitometric purposes, a 

 definite, timed routine of rocking is usually set up. The trays used are customarilj^ 

 deeper than the usual developing trays to permit steeper angles and thus secure more 

 violent washing effects as the developer sweeps across the emulsion surface. The fUms 

 or plates, as needed, are laid or fastened in the tray to hold them mechanically. A 

 typical cycle consists of tipping the tray to an angle of 30 to 45° in each of the four 

 possible directions, completed three times each minute, thus giving verj^ complete 

 agitation, and renewal of the developer in contact with the emulsion surface. 

 Attempts have been made to substitute mechanical rocking, for the hand system, but 

 these have been unsuccessful owing to the rather peculiar difficulty that the}'' were too 

 regular. Certain preferred paths tended to appear, with resultant systematic streak- 

 ing or unevenness. The slight irregularities of even the best hand systems appear to 

 be enough to eliminate these, and hence net a better result. 



