60 Trans. Acad. Set. of St. Louis. 



cellence. These lines might be considered to be contour 

 lines surrounding the summit of a surface. This summit, 

 representing the maximum of excellence of negatives would 

 be on the vertical axis, £J, and very close to the origin. It 

 would correspond to normal conditions for dark-room work. 

 The surface would sink to a minimum along the zero line 

 shown in the diagram, and would then rise again in the im- 

 mense field representing the conditions under which positives 

 may be developed. The conditions of maximum excellence 

 for positives are as yet unknown, but the best pictures yet 

 obtained, which seem to be as near perfect as could be wished, 

 had exposures of two and a half minutes in strong diffuse 

 light just outside of direct sunlight at a south window. This 

 illumination was probably about 400, on the scale used in this 

 paper. The pictures were developed at the same point. 



In the pictures here presented the plates have all received 

 uniform treatment. No shading of highlights has been done. 

 In the etching during the half-tone reproduction all parts of 

 the plate have been treated alike. 



In reproducing Fig. 6 it was found that on account of a 

 muddy background effect in those parts of the plate which 

 were about to reverse, the original did not submit itself 

 readily to the half-tone process. Details which could be 

 clearly seen could not be satisfactorily reproduced. The 

 plate was therefore re-photographed by ordinary means, and 

 from this plate a print was made which has been reproduced 

 in half-tone. 



This plate having an exposure of 3200, marks the beginning 

 of reversal. All exposures greater than this lie above the 

 horizontal zero line. The picture does not wholly reverse 

 until the exposure has reached 120000. This broad belt of 

 mongrel effects extends along the whole length of the hori- 

 zontal zero line, from dark-room conditions to critical illu- 

 mination. The upper limit of this belt will of course vary 

 with different plates, depending upon the density of the plate 

 in the deepest shadows. 



So soon as the plate is developed in a light stronger than 

 the critical value, no mongrel effects appear, and the expo- 

 sure time for a plate yielding zero effects drops at once to 



