526 HANDBOOK OF PHOTOGRAPHY 



scenes, are photographed. The condition is usually worse with plates than with film 

 negatives, because the greater thickness of plates produces a larger area of 

 halation. 



3. Prevention. — The use of films or plates having light-absorbing or "antihalation^' 

 backing will usually prevent halation. Films usually produce less objectionable 

 halation than plates. In processing films or plates likely to show halation, give full 

 exposure and full development. The developer should contain initially a small 

 amount of accelerator, and the amount of accelerator should be increased in small 

 amounts, gradually, as development proceeds. 



4. Remedy. — Local reduction or reto\iching may help sometimes. 



Fading. 



1. Distinguishing Characteristic- — Brown, yellow, or sepia tones in the negative, 

 appearing some time after processing. 



2. Cause. — Incomplete fixation, or, more usually, incomplete washing after fixing. 

 Use of old or exhausted fixing bath containing excess of silver. Insufficient agitation 

 when first immersing films in fixing solution. Tendency to stain increases with old 

 fixing baths which have become neutral or alkaline rather than acid and with strongly 

 alkaline developing baths. 



3. Prevention. — Complete fixing and washing. A rule which is often followed is 

 to fix the negative twice as long as the tjme required for the white opalescence to dis- 

 appear completely. Wash until all traces of hypo are removed. Make sure that the 

 fixing bath is acid. 



4. Remedy. — No remedy is available which is entirely satisfactory in all cases 

 although printing or copying the negative through a yellow filter has sometimes proved 

 quite successful. These yellow silver stains sometimes may be eliminated by remov- 

 ing hypo by washing thoroughh'^ in water, and dissolving sodium thiosulphite with a 

 1 per cent solution of potassium cyanide in which the negative is washed. Potassium 

 cyanide is a deadly poison and gives off poisonous fumes of hydrocyanic acid. 

 Extreme care should be taken to keep this solution from the mouth and to prevent 

 breathing cyanide fumes. The operation with potassium cyanide should be carried 

 out in a well- ventilated room. The film should be removed from this bath as soon as 

 signs of reduction of the image appear and should be washed thoroughly in clear water. 



Finger Marks. 



1. Distinguishing Characteristic. — Finger marks, frequently showing Bertillion 

 fingerprint lines. 



2. Cause. — Impressing moist, wet, dirty, or greasy fingers on the wet or dry nega- 

 tive. Damage is most easilj' done when the negative is wet, especially if the finger 

 print is made on the emulsion side of the negative, (a) Marks showing the lines of the 

 fingers are most frequently caused by handling the dry negative with dirty, moist, or 

 greasy fingers, ih) If the dark marks show mereh'^ an outline of the finger, the prob- 

 able cause is that of handling the negative with wet or greasy hands, (c) If the finger 

 marks are dark, contamination of the fingers with developer is indicated, {d) If the 

 finger marks are light or semitransparent, contamination of the fingers with hypo is 

 indicated. 



3. Prevention. — Cleanliness and care are the best preventatives, as these defects 

 are due only to lack of cleanliness. 



4. Remedy. — -Grease fingerprints impressed on a dry negative may frequently be 

 removed by wiping negative with lens tissue or soft lintless cloth dampened with 

 carbon tetrachloride. No remedv for other causes. 



