Nipher — Properties of Light-struck Photographic Plates. 153 



plate nearer to the lamp. If already fogged, a plate may 

 thus be cleared up in a very remarkable way. If the plate is 

 too near the light during the whole time, there is a loss of 

 detail. By allowing the developing to begin four or five feet 

 from the lamp, moving it up as necessity arises to within two 

 or three inches, and with a cool and weak developer, the picture 

 may be developed for an hour if desired. During this time the 

 details are coming out with continually increasing sharpness. 



When the spark length is less than twelve or thirteen milli- 

 meters, no disruptive and luminous sparks are seen on the 

 plate. There is a violet corona around the medal. The pic- 

 tures given by the positive pole show radial discharges, 

 bounded by a dark band, like a halo. For short spark lengths 

 of 5 or 6 mm., the halo is close around the medal, and it 

 increases in radius as the spark length increases. With a 

 spark length of about sixteen millimeters, a dark halo appears 

 distinctly on the plate before developing. This has been seen 

 only a few times. Thus far it has not been found possible to 

 save it. It washes out in the developer. It begins to fade 

 and an inner one, apparently midway between it and the medal, 

 begins to appear. The outer halo has disappeared, before the 

 inner one has fully developed. When the developing is ar- 

 rested at an earlier stage, the outer ring is lost in the fixing 

 bath. 



The shape of the dark halo conforms to the general shape 

 of the body. In some cases, where disruptive effects of ex- 

 ceptionally strong character have passed, their tracks are 

 shown on the negative. These tracks are in all cases dis- 

 tinctly broader and darker where they cross the dark halo, 

 than elsewhere. 



Figs. 2, 3 and 4 show some of the peculiarities mentioned. 

 In Fig. 2 the spark is delivered to the large Aveight, but the 

 smaller one is so near that it has very nearly the same poten- 

 tial as the larger. It is joined to the larger by a spark, which 

 practically unites the two bodies at each discharge. 



In Fig. 3 the smaller weight has a much lower potential 

 than that of the larger. It corresponds nearly to that of the 

 halo encircling the larger weight. In Fig. 4 the distance be- 

 tween the weights is still greater, and the smaller weight is at 



