Trowbridge — Magnetic Field and Coronal Streamers. 193 



solid until just before the X-ray stage; then it becomes hollow; 

 and at the X-ray stage it appears only at the instant the 



magnetic tield is excited as a violet light, and then becomes 

 indistinguishable in the fluorescence of the bulb. A violet 

 brush, however, persists on the surface of the anode at the 

 X-ray stage. "When the vacuum was increased to so high 

 a degree that the bulb could not be excited by a coil giving 

 a six-inch spark, the application of the magnetic field im- 

 mediately resulted in the production of the rays. 



When the cathode mirror of an ordinary X-ray bulb is 

 made the anode instead of the cathode, the current passing 

 in the usually unfavorable way for the production of the 

 rays, and at the same time is also the pole of a powerful 

 electromagnet, the bulb gives out X-rays in great abundance. 

 This is not the case when the magnet is not excited. The 

 magnetic field, therefore, causes the anode to produce X-rays, 

 probably by an increased energy of bombardment of the 

 platinum focal plane by the positive ions. 



Fig. 7 is a photograph of the X-ray bulb when the cathode 

 mirror has been made the magnetic pole. Without the excite- 

 ment of the magnetic field the bulb could not be made to 

 give the X-rays even with a coil producing a six-inch spark; 

 the vacuum having increased greatly during the previous 

 use of the bulb. 



When the field was excited, however, a brilliant fluores- 

 cence was produced, without any appearance of X-rays. The 



