TROWBRIDGE. — RONTGEN RAYS. 259 



of these is placed in a gas or under the conditious which are to be ex- 

 amined, while the other is protographed according to Feddersen's method 

 by a revolving mirror. A previous study of the behavior of various 

 electrodes had led me to select cadmium for the terminals of the spark 

 gap which are examined by the revolving mirror. The light from cad- 

 mium is very actinic ; and with carefully pointed terminals shielded from 

 the currents of air produced by the revolving mirror, very sharp photo- 

 graphs of electrical oscillations can be obtained. The resistance of a 

 spark in air or in rarefied media can be estimated by this method to 

 one half an ohm. 



The revolving mirror which I employed was the one I have used in 

 previous researches.* It is a glass concave mirror of ten feet focal 

 length, silvered on its concave surface, and corrected for the use to 

 which it was put. The improvements in electric motors and storage 

 batteries enables one to obtain great steadiness of rotation. The accora- 

 panying figure represents the revolving miri-or, together with the electric 

 motor and the chronograph, and the diagram explains the parts. Fig- 

 ure 4 is the apparatus in elevation, and Figure 5 in plan. M represents 

 the mirror, J5J the motor, (7 the chronograph, S the stylus, ^the cutting 

 tool for obtaining electrical contact at the instant the mirror reaches a 

 definite angle in its revolution. This cutting tool passes through a strip 

 of type metal, T. This strip is adjusted up and down by the screw 0, 

 and is adjusted in a direction at right angles by the spring bolt P. The 

 catch R releases P, at any desired moment, by means of a tension on a 

 string connected to it. The stylus S is drawn along guides by a string 

 which is connected through clockwork to a weight, W. A fan, F, serves 

 to control the movement of S. The stylus is also released by a catch 

 which can be tripped at any moment by the operator. 



The camera consisted of a box ten feet long, made of a trussed frame 

 covered with black cloth. This camera is shown in plan and eleva- 

 tion in Figure G. 3f is the revolving mirror, G the camera, S the 

 spark gap, P a partition which extends one quarter way through the 

 interior of the camera to shield the photographic plate at iV from the 

 direct light from the spark gap S. The photographic room in which this 

 camera is placed was about twenty-five feet square, and was provided 

 with shutters of orange fabric. In one corner of the room is the de- 

 veloping closet. The same room contains a mercury pump for exhaus- 



* Phil Mag., 1894 ; Am. Journal of Science, 1894 ; Velocity of Electric Waves, 

 Am. Journal of Science, 1895. 



