676 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



The distance from the platinum of the instrument to the centre of 

 the target of the tube was 22 cm. in all the experiments described in 

 this paper, with a few exceptions which are noted where they occur. 

 The metallic sheets were interposed in the beam of rays at about 11 cm. 

 from the platinum of the instrument in all cases. Throughout the 

 remainder of this paper these metallic sheets will be referred to as 



" screens." 



PlGCEE 3. 



The two electrodes at opposite ends of the bulb were of aluminium, and be- 

 tween them passed the rajiidly oscillating discharge from tlie secondary of tlie 

 Tesla coil. At the centre of the bulb was fixed the platinum target, upon which 

 the cathode stream from tlie larger aluminium electrode came to a focus. In 

 the ordinary use of the tube, the target had no metallic connection with the 

 rest of the system. The effect of the constriction in the walls of the tube in front 

 of the smaller aluminium electrode was to choke off more or less the cathode 

 stream emanating from that electrode. The tube was provided with a device for 

 reducing the vacuum. 



III. Approximate Measurement of the Energy. 



Soon after the instrument was set up, a test was made to ascertain 

 whether its deflections were proportional to the amount of radiant 

 energy which it absorbed per second at its sensitive surface. For the 

 purpose of this test, and of the calibration described below, the platinum 

 of the instrument had been coated on one side with a thin paste of lamp- 

 black in kerosene. A small battery-lamp using about 5 watts served 

 as a radiating source, and was placed about 3 meters from the in- 

 strument, the aluminium window of the latter being raised so as to 

 expose the platinum to the radiation. After noting the zero, the 

 current was sent through the lamp and the maximum deflection read. 

 Then the lamp was moved nearer and another deflection taken, and so 



