374 



Note. 



position. By means of the mechanism o, ]), q, the ^ focus tube 

 carrier /, can be adjusted in two directions, and all that is necessary 

 is to adjust the special Eontgen tubes held by the isolating clamps 

 m, m\ so that the small luminous spot is seen in the centre of the 

 screen surrounded by a dark circle, thus indicating that the central 

 ray of the cluster is following a path axial to the tube itself. _ One 

 can now regulate the desired sizes of the radiographic field indicated 

 by the extent of the illuminated zone of the screen, by sliding the 

 tube d nearer or farther away from the source of radiation. After 

 once centring the focus-tube by means of the incidence indicator, 

 this does not need to be repeated, the indicator being then slipped 

 out of the tube d and laid aside. 



Fig. 51.— Vertical Section of Complete Apparatus. 



Fig. 52. — Plan of Lamp Holder. 



All that now has to be done is, in the light of an ordinary dark 

 room, to place a small photographic plate, square by preference 

 and of very fine grain, on the centre of the leaden disk h, which 

 forms the base of the camera, and which is marked with a dia- 

 metrical cross for purposes of registration. The small object to be 

 radiographed is placed in direct contact with the sensitized surface 

 of the plate without the interposition of black paper. It only re- 

 mains to pull down the cylindrical camera body into its groove and 

 to allow the appropriate rays of a Eontgen tube, with a very small 

 anti-cathode, to act for a convenient time in order to obtain, thanks 

 to the normality of the incident rays and to the suppression of the 

 paper envelope, the great clearness of detail, which allows of the 



