36 



sheet of vellum, mounted on a frame, so as to form a " fluorescent 

 screen." 



Substances placed between this screen and a focus tube emit- 

 ting X-rays cast shadows, the density of which would depend upon 

 the thickness of the object and upon its permeability to the rays. 

 When permanent results were required a photographic plate was 

 used. This was placed in a light-tight bag or holder, made of 

 paper or other substance permeable to the rays, which then acted 

 on the sensitized film after passing through the paper, &c. The 

 plate thus gave a permanent record of the shadows which might 

 be seen on a fluorescent screen, the eftect on the plate depending 

 upon the permeability of the different parts of the object placed 

 between the plate and the tube. 



The permeability of a substance to X-rays differed from its 

 transparency to ordinary light. Glass, for instance, was some- 

 what opaque to the X-rays ; while diamonds were transparent or 

 permeable. The metals were on the whole opaque, aluminium 

 being the most transparent. Vegetable substances were trans- 

 parent to the rays. A metal object, placed in a wooden box, 

 would, therefore, give a shadow on the screen. Bone, owing to 

 the lime salts contained in it, was more opaque or less permeable 

 than flesh ; for this reason it was possible to obtain shadows of 

 the bones of men and animals through the flesh and skin. This 

 fact had led to the use of the rays for medical and surgical pur- 

 poses, and constituted the most important application of the rays, 

 as it enabled us to make investigations in the living body which 

 had never been possible before. Each substance had, therefore, 

 its particular or specific permeability to the X-rays, as it had a 

 specific resistance to a current of electricity, or a specific gravity 

 or conductivity for heat. And the fact that this permeability was 

 different for different substances enabled us to make use of the 

 rays for scientific investigations ; we could study the internal 

 structure of a body, or measure and localize hidden objects. 



The Lecturer then showed a number of lantern slides to 

 illustrate the applications already described. These consisted 

 mostly of photographs of X-ray pictures of shells and animals 

 and other radiographs taken for surgical purposes. A picture of 

 a large shell of the nautilus type showed the internal structure 

 with the successive divisions built by the animal as it grew. 

 Another picture, of a rat, showed all the bones in every detail. 

 The gradual growth of the bones was illustrated by a slide show- 

 ing three elbows taken from children of different ages with one 

 of an adult for comparison. This illustrated the use of the 

 rays for the study of the growth of the bones in men and animals. 

 The way in which bones are broken was illustrated by a slide 

 showing three radiographs of fractures of the femur, taken at 

 the Children's Hospital. A large number of prints from plates 



