X-rays and localisation / by James Mackenzie Davidson.

  • Davidson, James MacKenzie.
Date:
[1902]
Catalogue details

Licence: In copyright

Credit: X-rays and localisation / by James Mackenzie Davidson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    of sufficient density could be obtained. But there is another and more striking diiference between the action of light upon a photo- 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-tays 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 Eontgen 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.] -r\ tn- J1 f I am pleased to be able to express my thanks to Dr. Fmdley, lor his help in carrying out some of these photographic experiments.