Photomicrography 47 



once a minute and drained for 10 sec before being replaced in the devel- 

 oping solution. 



Hardening. The process of development, after it has continued for the 

 time specified by the manufacturer, should be abruptly terminated by 

 transferring the film to a weak acid. Two per cent acetic acid is commonly 

 used. It is much better to use a 3 per cent solution of sodium sulfate 

 in 2 per cent acetic acid. This hardens the gelatin, which not only pre- 

 vents mechanical damage but shortens the final drying time. Half a 

 minute in plain acid, or 3 min in hardener, is standard. 



Fixation. The image now consists of a mixture of silver and silver bro- 

 mide. The silver bromide is dissolved in an acid solution of sodium 

 thiosulfate, often miscalled "hypo." A typical solution contains about 30 

 per cent sodium thiosulfate with 2 per cent of sodium sulfite and 5 

 per cent of sodium metabisulfite. Most people find it easier to buy 

 the mixture ready prepared as "acid-fixing salt." 



The unwanted silver bromide is not directly soluble in the fixer. A 

 complex double salt is first formed and this is then dissolved out by the 

 fixer. The first stage is ended when the film passes from a milky to a trans- 

 parent appearance. The termination of the second, more important stage 

 cannot be seen. It is usual, therefore, to leave a film in fixer for about 

 10 min even though it appears clear after 3 or 4 min. 



Washing. Every trace of sodium thiosulfate must be removed or the 

 film will discolor rapidly. Twenty minutes in running water is sufficient 

 for most films. If the film is washed in a tank the stream of water must 

 be taken through a tube to the bottom of the tank. Merely running a tap 

 onto the surface is wholly inadequate. 



Drying. All film should be rinsed in a weak wetting agent before being 

 dried. This prevents water from gathering in droplets over the surface 

 and leaving ring marks as it evaporates. Some wetting agents are damag- 

 ing to photographic images and it is therefore best to buy some com- 

 pound, such as "Photoflo," specifically marketed for the purpose. 



Summary 



Processing negatives consists, therefore, of the following steps: 



Roll film ( step 1 only in darkroom ) : 



1. Load film into tank in darkroom (Fig. 51). Put lid on tank. 



2. Pour in developer (Fig. 52), at temperature recommended by manu- 

 facturer, and leave for time recommended. Invert tank once a minute. 



3. Pour developer back into bottle. Pour acid hardener into tank. Invert 

 once or twice, then leave 3 min. 



4. Pour off and discard acid hardener. Pour in acid fixer. Invert tank 

 once or twice. Leave 10 min. 



