Nipher — Properties of Light-struck Photographic Plates. 161 



Fig. 10 is a reproduction of a portion of a negative show- 

 ing ball tracks magnified about one hundred diameters. 

 While these tracks were being traced, disruptive sparks were 

 passing continually over the plate, and the tracks appeared 

 somewhat obscured when viewed with the unaided eye. A 

 pocket lens showed well defined tracks, and in the enlarged 

 photograph the blurred effect has entirely disappeared. 



This picture has been reversed twice, and shows the tracks 

 in black as they appear on the original negative. 



The fact that greatly over-exposed plates may be developed 

 in the light, was suggested by the fact that in exposure to 

 light during the taking of an electrograph, the electrical action 

 was annulled. Finally when a plate which at first promised 

 well began to fog in the dark room, the light of an incandes- 

 scent lamp was turned on, and the plate at once cleared in a 

 most remarkable way. 



This again suggested the idea of developing X-ray pictures 

 in the light. This has also been done with very satisfactory 

 results. Light-struck plates were used for this purpose, 

 which had been exposed for a day to the diffuse light of the 

 laboratory. Singularly enough these pictures were negatives 

 when they were inclosed in black paper during the X-ray 

 treatment, and they were positives if they were exposed to 

 the light while the X-ray was acting. 



The advantage of being able to study an X-ray picture 

 during the operation of the developing is sometimes very 

 great. The operation may then be pushed until the desired 

 features have been brought out, and it may be arrested before 

 they are obscured by over-developing. 



When the X-ray is thrown upon a plate in a camera while 

 an ordinary picture is being taken, all exposed parts of the 

 plate are affected alike. The action of light and of the 

 X-ray are added. If a picture be taken of a diagram in 

 black on white cardboard, the action of the X-ray will be 

 shown equally on the dark and the light parts of the image. 

 This is made evident by shielding half of the plate from the 

 X-ray by a screen of metal or of lead glass. There is a 

 marked difference between this result and that found for the 

 superposition of light and electrical action, as is shown in 



