584 l*rof. 11. AV. Wood on Plioloaraphic 



feeble and o£ long duration in order that reversals may be 

 obtained. My plates appear to indicate tliis qualitatively, 

 but quantitative data could doubtless be obtained with suit- 

 able apparatus. As I shall show later, it is possible to 

 administer the shock in such a manner that it comes out re- 

 versed even when the fogging-light is the flash of a single 

 spark. 



I have tried to obtain some idea o£ the action of the light- 

 shock by attempting to transform its effect on the plate into 

 an effect similar to that produced by ordinary exposure, by 

 means of the action of various chemical agents. These 

 experiments were all failures, but the interesting fact was 

 ascertained that an ordinary exposure appeared to be trans- 

 formed into a shock exposure by the action of a dilute bath 

 of bichromate of potash slightly acid with HNO3. This effect 

 is shown in fig. 4, Plate XXVI. A series of spark-images was 

 impressed on the plate, and then a series of images obtained 

 by illuminating the plate with the light of a candle shining 

 through a slit in a piece of black paper. One half of the 

 plate was then dipped into the bath, washed and dried, ex- 

 ])0sed to the light of a candle, and developed. A print from 

 this plate is reproduced in fig. 4, Plate XXVI. On the upper 

 portion, which was treated with bichromate, both the spark- 

 images and slit-images appear reversed, on the lower the 

 latter are not reversed. This experiment merely shows that 

 a plate which has been exposed to light in certain places and 

 then treated to the bichromate solution, is less sensitive to the 

 action of subsequent illumination on the Sf)ots which have 

 previously received light. The condition may appear at first 

 sight to be similar to that produced by a light-shock, but 

 there is in reality probably no connexion between the two, 

 for while light-shocks not followed by foggino- can be de- 

 veloped as not reversed images, the " bichromatized images " 

 do not develop at all unless the plate is fogged before 

 development. 



The nearest approach which I have been able to make to 

 the transformation of the effect of a light-shock into that 

 due to ordinary exposure, is by the action of the X-rays. 

 It was found that spark-images could not be reversed under 

 any circumstances if the plate was fogged by^ these rays 

 instead of candle-light. To prove that the case was not 

 analogous to the one in which ultra-^dolet light failed to 

 give reversals, owing to the comparatively brief duration of 

 the illumination^ long exposures were made with the X-ray 

 tube at a considerable distance. Xot only were reversals 

 never obtained, but it was found that after a brief exposure 

 to the rays fogging the plate by lamp-lig'ht failed to reverse 



