Trowbridge — Triangulation by Cathode Photography. 245 



Art. XXYII. — Triangulation by means of the Cathode 

 Photography ; by John Trowbridge. 



Photography by means of the Rontgen rays seems already 

 to be of great importance in examining certain portions of the 

 human body to determine the presence of metallic bodies, 

 calcareous formations, and fragments of glass. The shadow 

 pictures as they are taken at present, however, do not give the 

 approximate position of the shots, for instance, embedded in 

 the flesh. They indicate only the line in which they are situ- 

 ated. It occurred to me that the principles of triangulation 

 could be applied with success to determine more exactly the 

 position of the metallic particles. I was led to this conclusion 

 by considering Rumford's photometer. This instrument, it is 

 well known, consists merely of a vertical rod placed opposite a 

 suitable screen of white paper. The two lights, the intensities 

 of which are to be compared, are placed in a fixed position, 

 and throw two shadows of the rod on the screen. From a 

 measurement of the positions of the lights when shadows of 

 equal intensity are thrown on the screen, an extinction of the 

 brightness of the lights can be obtained. Moreover, by measur- 

 ing the distance between the shadows, and by drawing lines 

 from them to the lights, the position of the rod throwing the 

 shadows can be determined. This position is evidently at the 

 intersection of these lines. 



I have used two Crookes' tubes with two terminals making an 

 angle with each other, and have employed a to-and-fro excita- 

 tion by means of a Tesla coii. A suitable screen of glass 

 shielded the sensitive plate first from one cathode and then 

 from the other. From the distance between the shadow 

 pictures of a shot, for instance, on the back of the hand and 

 from the position of the terminals the height of the shot above 

 the sensitive plate could be estimated. It seems to me that 

 this method promises to be of importance in the surgery of the 

 extremities of the body ; for the question whether to make an 

 incision from the palm of a child's hand or from the back of the 

 hand is an important one. Stereoscopic pictures can also be 

 obtained. 



The use of a Tesla coil in obtaining shadow pictures is 

 advantageous in certain respects, for by changing the size of 

 the spark gap in the primary circuit of the Tesla coil one has 

 a great range of electrical energy at command. This range 

 can be still further increased by putting the spark gap in a 

 magnetic field. I have taken such pictures in less than a 

 minute, showing the bones in the fingers. The tubes were, 



