REGISTRATION BY PHOTOGRAPHY 63 



their influence should disappear in the total 

 impression received by the plate, even after 

 a very short exposure. I contrived, besides, 

 to eliminate even still more completely this 

 cause of perturbation, by repeatedly alterna- 

 ting the experiments, as I will proceed to 

 show. 



Fig. 4 represents a horizontal section of 

 the apparatus employed. AB is the photo- 

 graphic plate, 1 3 cms. wide ; E is the spark 

 enclosed in a cardboard box, FGHI, open 

 only on the side facing the plate, and allowing 

 the spark to act on one half, OB, of the plate 

 only ; CD is a lead screen wrapped in wet 

 paper, rigidly connected with the frame which 

 holds the plate. The " N " rays, proceeding 

 from any source, form a pencil, having the 

 direction NN'. With this arrangement the 

 " N " rays are arrested by the screen CD ; 

 the spark, while it acts on half- plate OB, is 

 sheltered from the rays. 



Now impart to the frame containing the 

 plate a translation to the right equal to half its 

 length (Fig. 5) ; the other half, AO, of the 

 plate takes the place formerly occupied by 



