PROGRESS IN RADIOGRAPHY. 157 



are designated in the illustration by the letters p // for the tube L. and //' //" 

 for tube L'.) 



Then if we can determine exactly the position of the two sources of light 

 on one hand, and, on the other, find the exact centers (I V) of the two shadows 

 on the plate, we can stretch two lines— say, of fine wire— from the center of the 

 lights to the center of the shadows; and these two lines will intersect at a point 

 in space which represents exactly the center of the projectile 



Suppose, now, that the two radiographs are completed. We can withdraw the 

 head of the subject without disturbing in the least the relations of the compass, 

 the Crookes tubes, and the photographic plate. Then we may stretch our two 

 wires from the tubes to the shadows and obtain with the most perfect exact- 

 ness the position of the center of the i)rojectile witli reference to the face of the 

 subject, or, what amounts to the same thing, with reference to the extremities 

 of the three branches of the compass, which represent the position of the face 

 in space wliile the radiograplis were being taken. 



Finally, if we attach to the body of the compass an adjustable needle which 

 will exactly indicate the intersection of tlie two wires (the center of the ball) 

 we will have all tlie elements necessary for finding the hidden missile. 



Such is the theory. In practice it is, of course, necessary to have 

 a special apparatus adapted to all heads, which through its perfect 

 rigidity insures the maintenance of the I'elative positions of the 

 [tranches of the compass, the Crookes tubes, and the photogi'ai)liic 

 plate. 



As this article is not a comprehensive treatise on radiography we 

 will not attempt to explain any of the essential operations — the 

 marking on the subject's face of the points touched by the compass, 

 the exact determination of the center of light of the X rays in order to 

 place the wires, or the adjustment of the articulated fourth branch 

 of the compass Avith its needle end. 



When all these oj^erations are accomplished the compass, with its 

 three stationary branches and its needle indicator, will, as we have 

 said, show the exact position of the projectile, 'i'his instrument 

 might be used to guide the operation of extraction, but in practice 

 it is replaced by an almost exactly similar " compass of extraction," 

 which may be sterilized and therefore more safely guide the surgeon 

 in his work. 



The patient is etherized, the operation compass is applied so that 

 its three branches correspond exactly witli the three |)()ints marked 

 (>n his face. Then the needle indicator on the fourth ami shows to 

 the surgeon (a) in Avhat direcHon he nuist cut to find the projectile. 

 th) by the distance between the sliding adjustment and the needle 

 lH)int how deep he must go to extract it. To simi)lify the opei-ation, 

 the compass has two needles, one regulated by the other. This allows 

 the surgeon to choose the most suitable \w\ui for an incision. This 

 point decided, the scalp is cut away and tlie skull laid ban' and 

 opened by trepamiing. 



As many times as he may need its guidance the surgeon can i)lace 

 the sterilized compass on the subject's head: and. finally, when the 



