124 Mr. James Mackenzie Davidson [April 25, 



WEEKLY EVENING MEETING, 



Friday, April 25, 1902. 



Sir James Crichton-Browne, M.D. LL.D. F.R.S., Treasurer and 

 Vice-President, in the Chair. 



James Mackenzie Davidson, Esq. M.B. CM. M.B.L 



X-Rays and Localisation. 



At the end of the year 1895 Professor Kontgen announced that he 

 had discovered a new kind of rays coining from a Crookes' Tube, 

 which he called the X-rays. This discovery created a widespread 

 interest in all parts of the world and among all classes of peoiDle, 

 and the reason is not far to seek, because the fact that these rays 

 have the power of passing through our bodies and casting shadows of 

 our bones upon a photographic plate or a fluorescent screen, at once 

 gave the discovery an intensely human interest. 



It is my privilege to-night to tell you something about these rays 

 and their application in surgical practice. At the risk of being too 

 elementary, I nevertheless purpose beginning with a short historical 

 sketch of the scientific discoveries and developments which led up to 

 their discovery. 



We will begin with the electric spark. Two types of instruments 

 are employed to produce disruptive electric discharges ; the Wims- 

 hurst or Holtz Influence Machine which, by merely turning a handle 

 produces electricity at a high pressure. The other instrument is an 

 induction coil, and it is the latter which I will use to-night in carry- 

 ing out my experiments. 



The Induction coil consists of a coil of insulated thick wire 

 bound round a bundle of soft iron wires, such as I show you here. 

 This is called the primary. It is introduced into the interior of a 

 large coil, consisting of a great length of very fine insulated copper 

 wire ; this is called the secondary. In this instrument here to-night 

 there are about nine miles of fine copper wire. The action of this 

 instrument is based upon the discovery made in this place by that 

 great philosopher and experimentalist, Faraday. If a current of 

 electricity be passed through the primary coil at the moment of 

 starting it, there is a current induced in the secondary coil in a 

 reverse direction ; and when the current is broken in the primary, 

 there is induced a powerful reverse current in the secondary ; so that 

 at each make and break of the current in the primary we have two 

 currents correspondingly induced in the secondary in opposite direc- 

 tions, but at a much higher potential. 



The current induced in the secondary when the current is broken 



