56 Trans. Acad. Sci. of St. Louis. 



the value of /in lamp-meters is ^2 = 0.25. The vertical axis 



E, where /=0, corresponds to r = oo . This vertical axis 

 therefore represents increasing exposures in the dark room. 



For all values of /less than .^ .>. .2 = 0.238 the picture be- 

 gins to reverse when the exposure time is 3200 seconds, 

 exactly as has been described for dark room development. 

 This value of / is a critical value. When the developing 

 plate has this illumination, a zero plate is obtained 

 for all exposures between 53 and 3200 lamp-meter-seconds. 

 The zero line which for smaller values of / was the horizontal 

 line Ea, of the diagram, drops straight down from a to h. 

 As this line is approached from the negative side, the picture 

 becomes more and more obscure, and on reaching it the plate 

 is blank, with the exception of a few faint isolated features 

 here and there, some of which appear to be positive and some 

 negative. If / is made slightly greater than this critical 

 value, the picture wholly reverses and becomes positive as 

 soon as E exceeds 53 seconds. For slightly smaller values 

 the picture is a poor negative. The line 6 6' is a sharp line 

 of separation between positive and negative results, and no 

 mongrel pictures are produced in the transition. The line 

 h h' is not horizontal. When the picture is developed at a 

 distance r = 1 meter, for which / = 1, faint positive pictures 

 are obtained with an exposure of 25 lamp-meter-seconds. 

 Thus far it has been found impossible to develop any nega- 

 tives in this light. The zero line evidently approaches the 

 axis / for illuminations greater than the critical value / = 

 0.238. For daylight development the picture evidently starts 

 from a positive condition, just as for small values of /it 

 starts from a negative condition. When the plate is illumined 

 with the critical illumination there is probably some condition 

 of chemical instability which should render the plate photo- 

 graphically sensitive to feeble influences which under other 

 circumstances might have no discernible effect. This might 

 apply to electrical oscillations. This critical condition has 

 been very carefully studied photographically, and plates have 

 been produced along the entire range represented by the line 

 a 6, both on the negative and on the positive side. 



