Nipher — Relation of Direct to Reversed Photographic Pictures. 57 



The area on the diagram representing the conditions where 

 good photographic positives can be produced has not yet been 

 adequately explored. An attempt was made to form an 

 exhibit of developed plates which would show by inspection 

 the results obtainable with various exposures E, and illumin- 

 ations /of the plate while developing. The plates were laid 

 upon a large table at points determined by the co-ordinates 

 E and /. In order to properly represent dark-room work, the 

 scale of E should be at least one meter for one lamp-meter- 

 second. Ordinary dark-room work with ordinary over- 

 exposures would then require a table a few meters in length. 

 The time of exposure which will yield good positives has, 

 however, been found so large that the plan proved impracti- 

 cable. Fig. 15, Plate 9, is a reproduction of a picture 

 which had an exposure of 16 hours to diffuse daylight. The 

 value of E in the diagram was about 16 X 3600 X 200 = 11,- 

 520,000 lamp-meter-seconds. This would be equivalent to a 

 continuous exposure of four months to a 16- candle lamp at a 

 distance of one meter. The position of this plate on the 

 exhibition table would be at a distance E = 11,520 kilometers 

 or about 6,900 miles from the axis /. The plate was developed 

 in a glass tray in diffuse daylight with reflected light thrown 

 up through the bottom of the tray. The value of / was 

 therefore over 200, which is about 800 times the value that 

 could be represented in the diagram. With these long ex- 

 posures the best results have been obtained by reflecting light 

 through the bottom of a glass tray, while the plate is being 

 developed. If this cannot be done, the plate should be lifted 

 out of the liquid at intervals, and the bottom should be exposed 

 to the light. Fig. 16, Plate 9, shows a trace of two ribs in 

 the bottom of the developing tray which cut off part of the 

 light. 



These long exposures show wonderful detail in the darker 

 shadows. They show with clearness details that are barely 

 distinguishable in the original plate from which the printing 

 was done. Kef erring to the plates, 



Fig. 1 is a positive, from which all the printing was done. 



Fig. 2 is a negative, printed from 1, with an exposure of 

 one lamp-meter-second, E=l, and developed in the dark 



