235 



between the points which were used as electrodes. The picture of this 

 stream was surrounded by interference or diffraction fringes, in some ways 

 resembling the fringes about the solid points themselves. 



The principal object of the present investigation was to study the 

 effect of a magnetic field upon this stream and to study the character of 

 the particles composing it. 



The apparatus used was that constructed by Prof. Foley and Mr. 

 Haseman for the investigation of interference fringes about a point dis- 

 charge, air streams, and vapor streams. It consisted of a wooden tube 

 G.ST meters long (Fig. 1). One part 20.3x20.3x280 cm. was made to tele- 

 scope over another part 15.2x15.2x457 cm. This provided a means of sepa- 

 rating the two parts for adjusting the points and magnets. Another portion 

 (E., Figs. 1 and 3) containing a plate holder (F) was made to fit over 

 the end. The tube was painted a dead black inside, and at intervals 

 screens (Fig. 9) were placed throughout the tube so that no light 

 would be reflected from the sides. An opening was made in the 

 lower side of the tube beneath the points and through this opening 

 a magnet was introduced so that the lines of force were per- 

 pendicular to the direction of the line of discharge. During 

 the latter part of the experiment a similar opening was cut in the 

 top of the tube and a second magnet placed above the first one so that like 

 poles faced each other. Figs. 4, 5, 6 and 8, show the successive attempts 

 to increase the field strength. The end of the tube (C) was closed by a 

 cap which shut out all light except from a pin hole, as shown by Fig. 2. 

 A circular disc with holes of various sizes provided a means of regulating 

 the amount of light. A is a 90° arc lamp, the crater of which is focussed 

 on the pin hole by the lens B. 



Light was shut out of the tube by placing a piece of black card board 

 in front of the pin hole. When a photograph was to be taken, if the dis- 

 charge was a silent or brush, the slide (S) was drawn from over the 

 plate, and after the tube had come to rest, the card board was removed 

 until the plate was sufficiently exposed. In case of the spark discharge 

 which fogged the plate if exposed too long, the card board was first re- 

 moved and the exposure made by withdrawing the slide. 



The points first used were made of sharply pointed brass pins 0.6J 

 mm. in diameter and 3 cm. long. In the latter part of the experiment 

 the brass pins were replaced by steel millinery needles 0.70 mm. in 



