356 HANDBOOK OF PHOTOGRAPHY 



resulted in specialized processing machinery not encountered in the other branches of 

 photography. 



Moving Fihn. — In mechanical handling of motion-picture film, one of the simplest 

 and most obvious procedures is to secure agitation by the progress of the film through 

 the solution. In fact, this agitation can only be avoided by rack and tank or various 

 of the reel methods of developing film and is always present in any continuous develop- 

 ing machine. The development produced has its own peculiar characteristics when no 

 other agitation is used. It is particularly subject to streaks, as unequal relative 

 velocities are often set up with resultant nonuniform agitation across the film width. 

 Development products tend to sweep back along the film, producing nonuniformity 

 of development. This is particularly marked in the case of sensitometric tests, where 

 the exposures progress systematically from one area to the next. In this case rela- 

 tively large differences can exist depending upon the placing of the exposure on the 

 film, low density or high density first. This sort of effect has lead to tacit standardiza- 

 tion in developing sensitometric exposures in this type of equipment. By common 

 consent the low density end is to lead in passing through the machine. With the 

 random distribution of densities in pictures, this effect rarely takes that form, but 

 sometimes there are trails of high or low density back of low- or high-density areas, 

 respectively. 



This limitation of the quality produced by the method is closely allied to the too 

 regular agitation noted for mechanically rocked trays. Greater turbulence is usually 

 required. 



Low-velocity Circulation. — In addition to the agitation produced by the motion of 

 film through the developer bath, it is well-nigh universal practice to add a circulation 

 system which pumps the developer around through a feed or storage tank as well 

 as through the developing tank proper. The velocities produced by this system are 

 usually rather low, as its main purposes are control of temperature and use of a rela- 

 tively large bulk of developer to insure uniform quality. In designing such systems, it 

 is desirable to make the most of the agitation so set up and to adjust the system to 

 try to break up the tendency to produce streaks due to the film motion itself. 



High-velocity Jets. — The low-velocity circulation is better than none but cannot 

 always be relied upon to eliminate streaks. As an extension of this basic idea, high- 

 velocity jets have been introduced to scour the surface of the film and break up the 

 streamline currents formed by the film motion. These jets are operated under the 

 surface of the solution to avoid aerial oxidation, and in general must be close to 

 the emulsion surface to secure the desired effect. 



A variation of this idea, designed to develop plates or cut films uniformlj'^ has also 

 been suggested and tested. The bottom of the developing tray can be made double, 

 with small holes closely spaced drilled through the upper piece. A plate to be devel- 

 oped can be placed face down, supported a little above the tray bottom. Sufficient 

 developer is used to adequately cover the plate and a pump is employed to make each 

 hole direct a stream of developer against the emulsion surface. With some motion 

 of the plate to avoid areas of high development corresponding to each jet, very uniform 

 results were reported. 



This, in conjunction with general circulation and film motion, is probable capable 

 of the most uniform results commercially obtainable today. 



Squeegees. — Various designs of squeegees have been tested in the attempt to pro- 

 duce uniform development under developing-machine conditions. Fixed soft squee- 

 gees may be placed directly or obliquely across the line of film travel in contact or 

 nearly in contact with the emulsion surface. Thus they act as barriers to the devel- 

 oper set in motion by the film itself and reduce the tendency to streaked nonuniform 

 development. 



