172 ROYAL SOCIETY OF CANADA 



boration of tliis kiml. We ultimately abandoned tbe use of the oscillat- 

 ing discharge, as Ave found that with the direct discharg-e a greater 

 amount of power could be employed, and more brilliant effects secured, 

 without risk of perforating the tube. The one-electrode method of 

 Tesla, though attended with less risk of perforation, appeared to require 

 a very extravagant ex])enditure of power. 



The tubes in which the kathode rays were allowed to impinge on the 

 glass walls Avere liable to a serious defect. If the X-radiating surface 

 were large, as in the maltese cross tube of Crookes, a large volume of 

 rays could be produced, giving brilliant effects, and readily visible 

 through the human body, but permitting very poor definition. If on the 

 other hand, the glass surface were made small, in order to secure good 

 definition, very little power could be a]jplied without melting the glass. 

 Aluminum windows were tried, following Rontgen, but could not be 

 made permanently air tight. Such devices as the use of a continuous 

 air-blaist, or of oil or water for cooling the tube, besides lieing trouble- 

 some, were open to obvious objections. All these difficulties were met by 

 the discovery of the "F(x-us Tube." 



Application of the Focus Tube. — The use of the focus tube represents 

 the greatest practical advance which has been nuide on the method of 

 Eontgen. The tube is one of the usual Crookes series, in which the 

 kathode rays arc concentrated by means of a concave electrode on a 

 plate of platinum. It is generally used to illustrate the production of 

 heat by the kathode rays. It was discovered that this focus was a very 

 powerful source of X-rays, which proceeded in straight lines through the 

 glass, and were capable of casting very sharj) shadows, since they pro- 

 ceeded from a very small focal point. We found the first application of 

 this tube, as applied to X-ray work, in the British Medical Journal, of 

 March 21, 189(î. 



We were fortunate in possessing a very fine focus tube, with a bulb 

 five inches in diameter, and a very large concave electrode. The tube 

 was opened, and the platinum plate bent at a more convenient angle, and 

 the tube then sealed and re-exhausted. The results were found to be far 

 superior in clearness and density to any of those obtained with the glass 

 tube. The original platinum plate, however, was so thin that a hole was 

 melted through it. The tube was therefore again opened and a thicker 

 plate substituted. The limit of power Avhich can be applied to these 

 tubes de])enfls on the size and thickness of the platinum plate. The 

 largei- and thicker the plate, the greater the ])0\ver that can be dissi- 

 pated without overheating the plate and spoiling the tiibe. With our 

 l)articular coil and tube, the limit ai)pears to be between 60 and 70 watts 

 on the primar3^ The stage of greatest X-ray elficiency is a little beyond 

 the stage of greatest heat production. 



Method of Exhausting Tubes. — The method which we adojjted for 

 exhausting the tubes may perhaps be worth mentioning, as we found it 



