362 HANDBOOK OF PHOTOGRAPHY 



Oil Spots. — When heavy enough, oil or grease deposits on the surface of the emulsion 

 will prevent both development and fixation. Sometimes with lighter deposits, 

 only development is effectively prevented but fixation occurs, leaving a clear spot 

 in the emulsion layer. Under still other conditions, there may be a central, unfixed 

 area outlined by a clear line where fixation but not development took place. 



Of course, no one intends to let oil or grease come in contact with the emulsion 

 surface, but it has been known to happen in many different cameras and darkrooms. 



Spots from Desensitizing Particles. — Occasionally there are white spots due to desen- 

 sitizing dust particles reaching the emulsion surface during manufacture. These are 

 relativelj^ much less frequent in occurrence than those from the causes noted. 



Many black spots in prints and enlargements can be traced to white spots in the 

 negative used. Black spots from other causes are not so common as white spots. 



Pinhole Fog ^Ireos.— Protecting paper covers, supposed to be light proof, sometimes 

 have pinholes which produce local high fog areas. The cure is care in selecting mate- 

 rial for such protecting covers, even when they may be considered onl}^ temporary. 



Static Marks. — When handled rapidly, particularly under dry conditions, dis- 

 charges of static electricity are sometimes produced which may leave many different 

 tj^pes of marks. Some marks are easily recognized as branching or tree static, and 

 some are less distinctive as more or less sharply defined black dots. In these cases, it 

 is often difficult to distinguish these markings definitely from other possible causes 

 of black spots. 



Emulsion Aging.— Si&c^ spots are sometimes produced by aging of emulsions, 

 particularly when in contact with paper and stored under conditions of high tempera- 

 ture and humidity. 



Chemical Dust. — Many chemicals have been found which produce black spots in 

 the final picture. All unnecessary contacts with dust, dirt, and chemical powders are 

 to be avoided because of the danger from such contacts. 



These causes of black spots are not all associated directly with development. 



As distinct from spots, with their many sources, other than processing, streaks 

 rarely have other cause than some processing irregularity, and most frequently this 

 lies in development. 



The retarding effect of products of development act in several different ways to 

 cause streaks and irregularities of development. 



Eberhard Effect or Mackie Line. — If a dark area is adjacent to one of less density, 

 the products of development of the dense area diffuse out into the neighboring area and 

 retard development more there than at points more distant from the high density. 

 Thus the high-density area outlines itself with a line or band of reduced density. This 

 effect is modified somewhat by the position of the emulsion layer during development. 

 Thus, if the emulsion layer is horizontal in stagnant developer, a uniform effect all 

 around the dense area is expected. On the other hand, if the emulsion layer is vertical, 

 the effect may be more pronounced below dense areas than above, as the products of 

 development are, in general, heavier than the original solution and hence tend to set- 

 tle, affecting areas below the dense area more than above it. This may be traced as a 

 streak an inch or so long under some conditions of development. 



The effect of uniform motion of an emulsion layer, such as that produced in many 

 motion-picture developing machines, has an effect very similar to that just outlined, 

 except for the fact that in this case it is no.t gravity but the motion of the film through 

 the viscous developer that tends to sweep development products back along the film 

 and thus modify the development. For regularly arranged exposures, such as 

 sensitometric test strips, this effect has been studied considerably, and considerable 

 difference found at times. As a result, sensitometric exposures used in motion- 

 picture laboratories are usually developed with the toe or low-density end leading. 



