164 Tram. Acad. Sci. of St. Louis. 



a distinct trail. With a longer exposure it is eliminated. 

 The unit of exposure may be roughly considered as one candle- 

 metei -second. With a fixed illumination, the exposure may 

 be varied, by varying the time of exposure. It appears that 

 for any exposure, there is some definite degree of illumina- 

 tion in the dark room, which will yield what might be called 

 a zero plate. No picture will appear on it if lights and 

 shadows are each uniform on the object, as in case of a dia- 

 gram in black on white cardboard. This picture will become 

 a negative in a darker, and a positive in a lighter developing 

 room. With an exposure of half a second a plate which will 

 develop as a perfect negative in a proper dark room, will 

 develop as a zero plate if the room is dimly lighted. In the 

 parlance of the photographer, it will fog. In a still brighter 

 light it will develop as a positive. In this action there must 

 be a time co-ordination in the action of the developer and the 

 light of the dark room. With a given strength of developer 

 it appears from results thus far obtained, that a maximum 

 degree of excellence will be secured with a definite degree of 

 illumination in the dark room. The results thus far obtained 

 with half-second exposures, are by no means satisfactory, 

 considered as products of the photographer's art, but the 

 pictures of street scenes are distinctly positives. If results 

 comparable with those for longer exposures are attainable, it 

 involves a delicate adjustment of the illumination of the dark 

 room and strength of developer which has not so far been 

 secured. 



With an incandescent lamp burning in the dark room, it 

 is easy in half -second exposures to obtain a rather poor neg- 

 ative, by holding the bath in the shadow of an object eight 

 or ten feet from the lamp. By holding the plate in the light, 

 and going somewhat nearer, the same plate with the same 

 exposure, will yield a picture which is distinctly a positive. 



With a very much over-exposed plate, it is difficult to get a 

 room dark enough to yield a negative. With a very short 

 exposure, it is equally difficult to get positives, and only by a 

 very great illumination of the plate while in the developer. 

 The condition of zero plate when only the time of exposure 

 and the illumination of the developing room are variable. 



