128 Mr. James Mackenzie Davidson [April 25, 



graphic plate. He then excited the tube by the electric current, and 

 then developed the photographic plate. He found the image of his 

 strip of platinum, as he expected, but he also found a second strip 

 which he could not explain. On examining the door he found that 

 the unexpected strip came to be opposite the beading. It was a pine 

 door, varnished but unpainted. At first he wondered whether the 

 additional thickness of the beading would account for the unexpected 

 strip, but he found wood so transparent, that this view was untenable. 

 He then removed the beading from the door. The explanation was 

 then obvious. Some white lead had been used to cement the strip to 

 the door, and it was the white lead which by its density had caused 

 the shadow on the photographic plate. 



It is remarkable that Professor Eontgen, in his original com- 

 munication concerning the X-rays, described almost all the properties 

 which they are known to possess. Very little, indeed, has been added 

 during these seven years. The chief advance has been in the methods 

 of their application to surgical and medical work. 



I will now explain how the X-rays take origin in a Crookes' tube. 

 This rough model illustrates the path of the cathodal stream by 

 means of these cords. A curved cathode, similar in shape to an 

 ordinary reflector, is made of aluminium. The cathode rays emerge 

 from it normal to its surface and converge towards a point, then 

 proceed for a short distance in a straight line, and then gradually 

 diverge — wherever this cathodal stream impinges upon solid matter X-rays 

 take their origin. From what has been previously said, it would 

 naturally occur to you that X-rays must have been produced long 

 before they were discovered. This is so. Looking backward, we 

 now know that Crookes in his researches must have been producing 

 X-rays from his tubes. Lenard, no doubt, had X-rays intermingled 

 with the cathode rays which he had so laboriously got outside his 

 tubes. In the early tubes which Eontgen used, the cathode stream 

 was allowed to impinge upon the bottom of the glass tube. The con- 

 sequence was that the X-rays took origin from a more or less large 

 surface, and consequently the photographs produced by a tube of this 

 kind were necessarily rather blurred; but Professor Jackson, of 

 King's College, London, suggested putting in the interior of the tube 

 a metal anode, inclined as you see in this modern tube, and in this 

 way the converging cathode stream from the concave cathode impinged 

 upon the platinum target, giving rise to X-rays richly from a com- 

 paratively small point or surface. This model served to illustrate 

 what happens. The slide now thrown upon the screen illustrates the 

 importance of the proper distance of the anode from the cathode. It 

 should be so placed as to meet the cathode stream at its narrowest 

 part. The impact thus gives rise to a rich production of X-rays from 

 a point, and sharp shadows on the fluorescent screen or photographic 

 plate are thereby produced. 



A matter of some practical importance is that all parts of the 

 tubo which fluoresce green give ofi" X-rays to some extent. And 



