68 Trans. Acad. Set. of St. Louis. 



room. These conditions are represented by a point in the 

 diagram which is practically at the origin 0. 



Fig. 3 is a negative having an exposure S = 1000, and de- 

 veloped in the dark room. The point in the diagram is 

 marked d'. 



Fig. 4, is a negative having an exposure 1000, and devel- 

 oped at a distance r = 4 meters below a 16-candle lamp. The 

 illumination is 7=0.0625. The point in the diagram thus 

 determined is marked d. 



Fig. 5 is a negative having an exposure JE = 1500, and de- 

 veloped exactly like No. 4. The point in the diagram is 

 marked m . 



Fig. 6 is also a negative, having an exposure E = 3200, and 

 developed under the same conditions as Figs. 4 and 5. This 

 picture has just begun to reverse. The light on the walk just 

 beyond the pan, has begun to turn white. The picture is 

 rather dense, but the details are sharp. The bright strip 

 around the picture has also become lighter. Point k in the 

 diagram represents the conditions. 



Fig. 7. The plate here reproduced has had the same ex- 

 posure as the last, but it was developed one meter below three 

 16-candle lamps. Hence / = 3. The diagram does not extend 

 beyond the value /= 0.25. 



Fig. 8. This plate had an exposure U — 3200 like the last, 

 but the value of /= 100. 



Fig. 9. The exposure was E = 24000 and the plate was 

 developed in daylight where / = 200. The plate is wholly 

 reversed. 



Fig. 10. Exposure E = 36000, /= 200. This picture is a 

 clear positive, and was developed without any restrainer. 



Fig. 11. This plate had an exposure 60000, and was devel- 

 oped 2.25 meters below a 16-candle lamp. The value of 

 I = 0.197. This is somewhat less than the critical value of 

 /, represented by the line ab in the diagram. The picture 

 has only in part reversed, although the plate last described, 

 with an exposure only a little more than half as much, was 

 fully reversed, because of the larger value of I. 



Fig. 12. This plate had an exposure £J = 120,000. 



The value of / = 0.0625. The only part of the plate which 



