260 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion, and the wires from the storage battery are led to a switch board 

 in this room. A connecting room contains a Rowland grating. 



With the facilities at ray command, I set to work to investigate the 

 conditions which seem to be necessary to produce the Rontgen rays. 

 I have said that my previous experience in studying oscillatory dis- 

 charges had led me to select cadmium for the material of the terminals 

 of a spark gap. The light is liighly actinic and with not too strong 

 discharges, and vrith sharply pointed terminals well defined images of 

 even minute evidences of oscillations can be obtained. I have also ex- 

 perimented with various developers, and finally adopted Rodinol. This 

 developer works with great intensity, and quickly brings out the latent 

 images. The study of rapid electrical oscillations enables one to esti- 

 mate with considerable accuracy the merits of different developers for 

 instantaneous photography. With a definite electrical circuit consisting 

 of a known capacity (Leyden jar) and a known self-induction, together 

 with constant rate of speed of the revolving mirror, one can obtain 

 the time of exposure ; and the number of oscillations brought out by the 

 various developers is a guide to their relative values. 



Having at my command a battery giving a voltage of twenty thousand, 

 with an internal resistance of only one quarter of an ohm per cell, 

 and therefore capable of giving a very powerful current, I first studied 

 the behavior of Crookes tubes, which were directly connected to the 

 terminals of this battery. I speedily discovered that no Rontgen raj-s 

 could be obtained with a voltage of twenty thousand. On strongly heat- 

 ing the Crookes tubes they were filled with a pale white light, which 

 showed very faint bands in the green when examined by a spectroscope. 

 After a short interval, the entire strength of the battery appeared to be 

 manifested in the tubes, the electrodes became red-hot, the medium 

 apparently broke down, and offered little resistance to the battery cur- 

 rent. This white discharge showed, even at its culminating point, no 

 Rontgen rays, and appeared to be of the nature of a voltaic arc discharge. 

 I then employed the Plante rheostatic machine. I found tiiat at least 

 one hundred thousand volts were necessary to produce the Rontgen 

 rays, and that they were produced more intensely as I increased the 

 voltage, — certainly to the point of five hundred thousand volts. In order 

 to ascertain how great a loss occurred in this machine on discharging in 

 series, I investigated the length of spark obtained by varying the number 

 of Leyden jars in this machine. Experiments to be described later in 

 this paper had shown me that increase of electromotive force diminished 

 the resistance offered to a spark in air. The higher the electromotive 



