104 Dr. S. P. Thompson on Hyperphosphorescence. 



When sheets of paper or aluminium covered with these were 

 placed face down npon the sensitive film, so that the #-rays 

 were compelled to pass first through them, some results were 

 obtained tending to show that the method might have some 

 advantages : but the resulting negatives were always patchy 

 and irregular. The most striking effect, however, was quite 

 unexpected. Care had been taken to keep these prepared 

 sheets of fluorescent material in the dark for a sufficiently 

 long time for all visible phosphorescence or persistent fluo- 

 rescence to disappear. This, in the case of the sulphide of 

 calcium, required many hours. The powdered fluor was also 

 heated beforehand. Nevertheless, though no visible phos- 

 phorescence was present, the sensitive films were fogged by 

 rays emitted from these materials. Fluor-spar and the pla- 

 tino-cyanides did not produce any noticeable fogging, however. 

 Even after being kept six weeks in darkness the sulphide of 

 calcium is very active in the emission of rays that will affect 

 a photographic plate. 



While these experiments were in progress other experi- 

 ments were begun to ascertain if from any other sources, 

 such as sunlight or the light of the arc lamp, any rays could 

 be obtained having, like the .i'-rays, the power of penetrating- 

 opaque bodies. From the arc lamp, with an exposure of 

 about two hours, shadows of pieces of metal were obtained on 

 a photographic plate through a piece of pine-wood several 

 millimetres thick ; but aluminium was found to be totally 

 opaque to everything radiated from the arc and to sunlight, 



While the experiments on fogging were still in hand there 

 was published the observation of M. Henry on the effect of 

 sulphide of zinc in apparently augmenting the transparency 

 of aluminium to #-rays ; an observation which had an obvious 

 bearing on that which was under investigation. A number 

 of small portions of the fluorescent substances with which we 

 were experimenting were then placed upon the front of a 

 sheet of aluminium about 0'5 millimetre thick, behind which 

 was a gelatino-bromide plate (a Cadett's " lightning " plate) ; 

 and these were left for several days upon the sill of a window 

 facing south to receive so much sunlight (several hours as it 

 happened) as penetrates in February into a back street in the 

 heart of London. On developing the plate it was found that 

 behind those spots where portions of uranium nitrate and 

 uranium ammonium fluoride had been placed, photographic 

 action had taken place through the aluminium sheet. No 

 very distinct effect had been been produced by the other 

 substances. On communicating these observations to Sir G. 

 Gr. Stokes he drew the writer's attention to the similar obser- 



