1902.] on X-Bays and Localisation. 131 



of sufficient density could be obtained. But there is another and 

 more striking difference between the action of light upon a piioto- 

 graphic film and X-rays. Before saying more upon this matter I 

 will take a photograph by means of X-rays now. In this paper bag 

 there is a plate 20 inches by 24 inches. It is laid upon the table 

 and upon it is placed a metal design. The tube is now placed above 

 it, and, excited by the coil, produces X-rays. The metal stops the 

 passage of the X-rays reaching the films, but all the part not covered 

 by the metal receives the X-rays, because the paper envelope though 

 being quite opaque to light is transparent to the Rontgen rays. The 

 usual proceeding after the exposure is made is to take the plate into 

 the usual dark-room and develope it in the usual way, but I think 

 it would be more interesting to you if I developed this plate before 

 you in this room. I therefore, now that the exposure is complete, 

 take it out of its coverings, and it is now being exposed to this 

 brilliant electric light, and no doubt many suppose that with this 

 excessive exposure to ordinary light that the X-ray picture will be 

 fogged out and spoilt. But I hope that this will not be the case. 

 I now put it in the developing dish and allow the developer to flow 

 over it in the usual way, and if this experiment is as successful as 

 the rehearsal ones have been, I think you will see the photograph 

 gradually appear. The picture you see is the reverse of the usual 

 X-ray negative ; in the ordinary negative the part of the film that has 

 been protected by an opaque substance appears naturally white 

 because the silver salts are dissolved out by the fixing solution; 

 after development in this case the metal part is black and the unpro- 

 tected film is comparatively light. As far as my scanty leisure will 

 allow I have carried out a few experiments testing the effect of 

 X-rays and light combined upon various kinds of photographic 

 plates, and the explanation of what you have witnessed is given by 

 a slide which I will now show upon the screen. It is a remarkable 

 fact that if a photographic plate be exposed to ordinary light for a 

 considerable time and then exposed to X-rays for a definite time, that 

 the effect is entirely different to what occurs if the order is reversed, 

 that is to say, if a plate is previously exposed to X-rays for a 

 definite time and then to ordinary light. 



The next slide is from a negative where the exposures to X-rays 

 and to light were made in strips at right angles to each other. You 

 will notice the reversal produced by the X-rays throughout, the com- 

 bined action of X-rays and then light making the film lighter than 

 the separate action of either. It seems to me that these curious 

 effects are well worth a systematic investigation. [Slide; it came 

 up ordinarily, and then suddenly reversed and remained as now 

 shown.] 



I am pleased to be able to express my thanks to Dr. Findley, for 

 his help in carrying out some of these photographic experiments. 



K 2 



