52 Trans, Acad. Sci. of St. Louis. 



paper referred to, and it was clearly pointed out that devel- 

 opment in the light was the feature to which attention was 

 invited. 



In the present paper the conditions which yield direct and 

 reversed pictures will be given. The work has been restricted 

 to Cramer's * * crown ' ' plate, and the developer used was hydro- 

 chinon. The plates were all exposed in a printing frame 

 either to the light of an incandescent lamp or to daylight. 

 The pictures were all printed from the same lantern slide, or 

 positive, so that the direct pictures are all negatives, and the 

 reversed are all positives.* The over-exposed negative and 

 the under-exposed positive require the same kind of treat- 

 ment. A restrainer must be used, whose function is to keep 

 away the fog. The fog is incidental to an approach to a zero 

 condition in which the plate will be blank. The restrainer 

 does not change the character of the picture as regards posi- 

 tive or negative. It is not necessary to use it for what are 

 called normal exposures, when negatives are developed in the 

 dark room, nor for normal exposures when positives are de- 

 veloped in the light. The amount of restrainer used must 

 increase as the zero condition is approached. The amount 

 needed may be as great as a twelfth of the entire bath in ten 

 per cent, solution of potassium bromide, and this may be sup- 

 plemented by the addition of from two to five drops of satu- 

 rated solution of sodium hyposulphite. When the picture to be 

 developed is a landscape with modulated lights and shadows, 

 any exposure from normal to more than ten million times 

 over-exposed may be developed in the dark-room. As the 

 zero condition is reached, the strongest highlights will reverse, 

 and the other parts of the picture will locally reverse as 

 greater exposures are given, but without complete loss of 

 detail. There will be incongruities in light and shadow, and 

 each local detail will have at a certain exposure, a minimum 

 of distinctness. A picture in which the shadows are alike, 

 and likewise the lights, will develop a blank at the zero con- 



♦ This method of exposure was adopted in order to secure known con- 

 ditions of illumination. For some of the longer exposures, a 3O0-candle 

 Packard incandescent lamp was used, and was found very satisfactory. 

 This lamp was kindly furnished by the manufacturers. 



