JUL ifi 1901 



THE RELATION OF DIRECT TO REVERSED PHOTO- 

 GRAPHIC PICTURES.* 



Francis E. Nipher. 



In former papers in these Transactions f the author has 

 given a partial exposition of the results of developing photo- 

 graphic pictures in the light. These results were reached in 

 the course of a long series of experiments, in which the films 

 were acted upon by electrical discharge. It was found that 

 the most rapid plates might be exposed to daylight for a 

 week, and that contact electrographs of coins might then be 

 produced in a well-known way. It was also found that these 

 pictures might be developed in the light, and that for expo- 

 sures to electrical action with a Holtz machine for several 

 minutes, these pictures were negatives. The parts of the 

 film most exposed to electrical action came out dark when 

 developed either in the dark-room or in the light, but those 

 developed in the light were clearer and gave less trouble from 

 fog. The significance of this was not then fully realized , and 

 there remain yet many points to be cleared up by further 

 study. Since that time specially treated plates have yielded 

 negatives in the light from ordinary camera exposures and 

 they showed a marked improvement when the light was 

 turned on. But the method is not as yet under sufficient 

 control so that the results can be obtained except at rare 

 intervals. 



The results given in the former paper seem to have been 

 misunderstood by many, who have apparently supposed that 

 the author was not aware of the fact that photographic posi- 

 tives had long been known as a result of developing greatly 

 over-exposed plates. This was expressly stated in the first 



* Presented and read in abstract before The Academy of Science of St. 

 Louis, April 15, 1901. 



t Trans. Vol. X., Nos. 6 and 9. 



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