J!ev;:i':>i(L'< in >Sjjecfrinu I'hotoijruj.Jis. 



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the spark- iinacres. This seemed very remarkable, for it was 

 subsequently ascertained that X-ray images could be reversed 

 even when produced by h^ng exposure to feeble radiation, by 

 subsequent exposure of the plate to lamp-light. This eflPect 

 is shown in fig. 5, Plate XXVI. 



The spark-images were impressed first. The pla*"o was 

 then exposed in vertical strips to the action of X-rays for 

 varying lengths of time, the left-hand strip receiving the 

 longest exposure, while the right-hand strip was not exposed 

 at all. Following this came an exposure in horizontal strips 

 to lamp-light, the lower strip having the longest exposure 

 and the upper none at all. It will be seen that there is no 

 trace of reversal in the upper left-hand corner, where the 

 fogging is due almost wholly to X-rays, while reversed edges 

 appear on all of the sparks in the lower right-hand corner, 

 where the fogging was due to light : moreover, in the lower 

 left-hand corner, where the X-ray fog preceded the light fog, 

 the imao-es are not reversed. On iuvestioatino- the matter 

 further, I found that shocks administered by single powerful 

 flashes of X-rays were reversed by subsequent exposure to 

 lamp-light. In this case, however, the time element appears 

 to be without much influence, for images formed by long- 

 exposure to very feeble X-radiation reverse in the same 

 manner. This seems very remarkable, when we consider 

 the fact that exposure to these rays changes the condition 

 produced by light-shock in some manner, so that it is im- 

 possible to reverse it by further fogging. 



The reversal by X-rays is illustrated in fig. 6. A plate 

 was wrapped up in black paper and 

 exposed to the radiation for several 

 minutes, a vertical iron rod shielding 

 the centre strip of the plate. The 

 plate was then exposed to lamp-light 

 for different lengths of time in strips 

 perpendicular to the shadow of the 

 iron rod. On development it w\is 

 found that on the end of the plate 

 which had received the shorter ex- 

 posures to light, the central strip 

 came out lighter than the back- 

 ground, while on the opposite end 

 of the plate it was darker. At a 

 certain point near the centre of the 

 plate, all trace of the shadow of the rod 

 had disappeared, showing that exposure to X-rays for some 

 time, and then to light for a certain time, jiroduces an image 

 no blacker than the light alone would have produced. 



Fi<r. 



