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



Such a result is shown in Fig. 11, Plate 7. The penum- 

 bra which separates light and shadow appears then darker 

 than either the lighter or the darker areas adjoining. Never- 

 theless the whole figure of the monk shown in the foreground 

 is sharply differentiated from the background. The dense 

 foliage to the right of the picture is still a negative, while the 

 entire left half of the picture has reversed. When the expo- 

 sure time has increased to about ten minutes of daylight, or 

 120,000 lamp-meter- seconds, the last detail of the picture has 

 just reversed or is about to reverse. This part is the deep 

 shadow among the foliage in the right of the picture. In Fig. 

 12, Plate 7, the picture is all reversed, excepting a small area 

 of the darkest foliage. 



When the exposure has been increased to two hours, a 

 sharply defined positive, yielding a good print, is yet obtained. 

 It is, however, somewhat dense, and prints slowly. The pic- 

 ture from this exposure is reproduced in Fig. 13, Plate 8. 

 The exposure for this plate is equivalent to 7200X200= 

 1,440,000 lamp-meter-seconds, or over sixteen lamp-meter- 

 days. 



When instead of developing the exposed plate in the dark 

 room, it is developed at a distance r = seven meters below a 

 16-candle lamp, the plane of the filament being horizontal, a 

 similar series of pictures is obtained as the exposure time in- 

 creases. The picture begins to reverse exactly as in the dark 

 room, when the exposure in lamp-meter-seconds is 3200. All 

 exposures less than this give negatives, of surprising merit. 

 The positive or reversed pictures having a greater exposure 

 than 3200 cannot be distinguished from those made in the 

 dark room. In the diagram representing the conditions of 

 exposure and development, the co-ordinates are exposure U, in 

 lamp-meter-seconds, and illumination, /, in lamp-meters, of 



the developing bath. The value of /is -^^ where r is the dis- 

 tance of the bath from the 16-candle lamp. The value /is 

 laid off on the horizontal axis of the diagram, and the num- 

 bers representing distances r, ranging between 2 and 7 meters, 

 are indicated along the axis / at places on the scale which 

 those distances determine. For example where r = 2 meters, 



