CHAPTER X 



EXPOSURE 



THE quality of a negative depends very largely on the 

 accuracy of the exposure given, though errors discovered 

 before development can be counteracted to a large extent 

 by suitable modification of that process. It is, however, 

 impossible to give any very definite instructions to cover 

 all cases, the times required are modified by so many 

 different factors. Actinometers and sensitometers are 

 made to measure the actinic value of the light and the 

 rapidity of plates, but they are not satisfactory when 

 using colour sensitive plates and colour screens. 



Trial Exposures. When there is doubt as to the 

 exposure required the best way to decide on the time is 

 to make trial exposures in the following way. Arrange 

 the object and illuminant, and focus on the ground 

 glass. Make as near an approximation to the required 

 exposure as experience or appearance on the ground glass 

 permits, say 10 seconds. Then insert the plate, draw the 

 slide, and give an exposure of half the time thought to 

 be correct, that is 5 seconds. Push the slide in one quarter 

 of its distance and expose again for 5 seconds more. Then 

 push it in another quarter and expose a further 10 seconds. 

 Finally, push it in another quarter and expose 20 seconds 

 longer. On development for the correct time demanded 

 by the developer used, four strips of different densities 

 appear on the negative, corresponding to the different 

 exposures given to the four quarters of the plate, 5, 10, 

 20, and 40 seconds respectively. The time corresponding 



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