Nipher — Pfoperties of Liyht-struck Photographic Plates. 165 



certainly cannot be very different from an inverse proportion. 

 The experiments thus far made show also that with a long 

 exposure, the best results can be obtained by developing the 

 plate in the light, as a positive, while for very short exposures 

 the best results are attainable by developing as a negative. 



These conclusions may be modified by a variation of the 

 strength of the developer. The limits within which the va- 

 riables may change and yield results of commercial value have 

 not been determined with precision for positive pictures. 

 What has been said of pictures taken in the camera, may also 

 be said of X-ray pictures on plates not previously light-struck. 

 If two plates are exposed in the same way to the X-ray, and 

 one be developed in the dark and the other in the light, the 

 former develops as a negative and the latter as a positive. 

 Either may be converted into a zero plate by a change in the 

 illumination of the plate while in the bath, as has been pre- 

 viously explained. The more careful study of these subjects 

 is still in progress. There is ground for believing that the 

 treatment of a plate by a slab of plaster of Paris moistened 

 with peroxide of hydrogen, according to the method used by 

 Russell * may be of value in developing X-ray pictures in the 

 light. Work in this direction has not yet progressed suffi- 

 ciently to warrant any final conclusions. 



The superposition of X-ray pictures on electrographs does 

 not as yet reveal any effect of either agent upon the action of 

 the other. This has been done with fresh plates and with those 

 which had been previously light-struck. In these experiments 

 half of the plate was shielded from the X-ray by a heavy 

 plate of lead glass. The pictures due to the two sources 

 were superposed, and the two effects were added where sim- 

 ultaneously acting. This is also in marked contrast to the 

 action of light on the electrograph, as is shown in Fig. 1. 



The superposition of X-rays upon a plate in the developing 

 bath while in a dark room promises interesting results, but 

 so far this has not been done from lack of time. 



* Science, Marcb^SO, 1900, p. 491. 



