384 



2. CooLiDGE Tube, Radiator Type, for X-ray Work. 



In exhibiting this, the most speQJalized type of X-ray- 

 tube, the structural and physical points of difference with 

 the standard type of "Coolidge" electron type of X-ray tube 

 were discussed. Tho very much greater thermal capacity of 

 the anti-cathode in the exhibited tube, together with the 

 radiator, allowed the convergent bundles of cathode-rays to 

 be intercepted much nearer their exact point of convergence. 

 This narrowing down of the point of cathode-ray bombardment 

 made it possible to far more closely approximate a point 

 source for the resulting X-rays, thus giving a much more 

 critical definition in any radiographic projection. One very 

 great advance made possible by the new tube was stereoscopic 

 fluoroscopy. Stereoscopic radiography has long been possible, 

 but to examine on a fluorescent screen in a darkened room, say 

 a chest, in stereoscopic relief, w^th all movements, is a more 

 recent achievement. Tlie thermal capacity of the anti-cathode 

 of the tube in question has allowed the bulb to be made 

 smaller, only 3'75" in diameter. By placing two of these 

 tubes side by side, their focal spots would not be at a very 

 much greater distance than the normal separation of the eyes. 

 In stereoscopic fluoroscopy, the two tubes are placed side by 

 side behind the patient. In front of the patient is placed the 

 usual fluoroscopic screen. The two tubes are then excited 

 alternately at a frequency of, say, 25 per second. There will 

 be, therefore, on the screen alternately two distinct shadows 

 of the object being exam.ined. It is only necessary now to 

 hold in front of the eyes a reciprocating shutter, to block the 

 vision of the right and left eyes alternately, in such a way 

 that the right eye sees the image on the screen formed by the 

 left tube, and vice versa. The brain then combines the two 

 images, and reveals the object examined in stereoscopic relief. 



A. R. Riddle. 



Evening Meeting, April 8, 1920. 



3. The Amethystine Colouration produced in Glass by 

 Ultra-violet and X-ray Radiation. 



On August 14, 1919, exhibits and remarks were set 

 before the Society on the above subject (vide Trans, and Proc. 

 Roy. Soc. S. Austr., vol. xliii., p. 420). It had been sug- 

 gested in various quarters that the colouration produced was 

 always due to ultra-violet radiation as from solar sources, etc., 

 and not due to X-ray radiation, in spite of the marked effects 

 seen in X-ray tubes, especially of the electron type, where the 

 colouration was definitely associated with the field of X-ray 



