MOTION-PICTURE PHOTOGRAPHY 701 



or one-half a printer point. Positive gammas vary between 1.8 and 2.3, occasionally- 

 going higher. The average value is 2.10. The laboratory may vary the positive 

 gamma to smooth out screen contrast where the gamma of the negative is too high or 

 too low. 



The product of the negative gamma and the positive gamma is called the "over- 

 all" or "reproduction" gamma; its importance is that it expresses the ratio of the 

 brightness differences of the positive in relation to those of the subject. Owing to the 

 presence of stray light, both in photography and projection, there is a tendency for 

 the picture on the screen to have less contrast than the subject, unless the loss is 

 compensated for by making the over-all gamma greater than unity. Accordingly 

 it is common laboratory practice to work to over-all gammas not lower than 1.2. 



While most of the secondary features of development and laboratory operation 

 must necessarily be passed over here, one point in developing-machine design deserves 

 mention: the problem of overcoming directional effects in development. As the film 

 is fed through the machine, the products of development (mainlj- oxides and bromide 

 salts) tend to diffuse from areas of greater exposure into adjacent areas, in a direction 

 counter to the direction of travel. The effect is to reduce the density of areas adjacent 

 to and following regions of high exposure; thus a sensitometric strip passed through the 

 machine with the light end leading will evidence a different characteristic than with 

 the dense end leading. 



In order to overcome this distortion, methods of turbulation and agitation have 

 been devised to prevent the deposition of the development products which cause the 

 trouble. The developer may, for example, be forced through nozzles beneath the sur- 

 face of the fluid, and the resulting jets agitate the developer sufficiently to minimize 

 such irregularities. 



It has already been remarked that improvements in emulsions and lighting are 

 closely correlated with improvements in development and other phases of laboratory 

 operation. These relationships are illustrated by the work which has been done on 

 developers in recent years. The primary ingredients of a developer are the reducing 

 agent; an accelerator, which is usually an alkali; and a preservative, generally sodium 

 sulphite. With orthochromatic film a very active developer, of the type now termed 

 "dynamite soup," was necessarily used, since the film was slow and the light sources 

 inefficient. The strong alkaline accelerator tended to cause grain clumping in develop- 

 ment. As emulsions improved it became desirable to devise a more restrained devel- 

 oper of the borax type, such as is now in general use. This utilizes a weaker alkali, 

 borax (sodium tetraborate), in combination with an excess of sodium sulphite, which 

 has a solvent action on silver halides as well as preservative properties against aerial 

 oxidation. Slower developers of this type afford a much finer grain characteristic. 



The following are the compositions of the widely used negative and positive devel- 

 opers mentioned in Films and Film Characteristics: 



Negative Developers 



P'ORMULA D-76 



Elon 2 g. 



Hydroquinone 5 g. 



Sodium sulphite (desiccated) 100 g. 



Borax (granular) 2 g. 



Water to make 1 1. 



Formula ND-2 



Rhodol (metol or Elon) 2.5 g. 



Hydroquinone 3 g. 



Sodium sulphite (anhydrous) 75 g. 



Borax 5 g. 



Water to make 1 1. 



