76 DE. S. R. MILNER ON THE NATURE OF 



This displacement of the streamers by a magnetic field I found could be easily 

 observed without using a rotating mirror. Figs. 6, 7 and 8 are photographs of sparks 

 taken between the edges of two zinc plates placed at right angles to each other about 

 a centimetre apart, fig. 6 without, and figs. 7 and 8 with a magnetic field on, in 

 opposite directions in the two cases. In the two latter figures the streamers are 

 displaced along the edge of the plate at the bottom of the figure, and the amount of 

 drawing out being a little irregular some of them can be seen without overlapping. 

 From the upper plate seen end on the streamers come off at the side. By increasing 

 the inductance in the sparking circuit the separation of the streamers can be made 

 still greater ; with sodium electrodes and large inductance an enormous drawing out 

 (several centimetres) of the streamers at the side of the spark can be obtained, and 

 the discharge even made to pass in a spiral path between the two electrodes, which is 

 evidently due to the action of a magnetic field on moving ions. 



The next point which called for attention was the examination of the streamers in 

 the different monochromatic lights corresponding to the metallic lines of the spark. 

 The measurements of Messrs. SCHUSTER and HEMSALECH, as has already been 

 mentioned, were restricted to the case where no inductance was in circuit with the 

 discharge. Under these conditions the period of the oscillations is so extremely 

 minute that the streamers corresponding to the individual discharges are all super- 

 posed on each other, and the details of the structure of the single streamer are 

 completely masked. My first experiments were carried out on similar lines, but with 

 inductance inserted in series with the spark. Although they showed some interesting 

 features, the streamers corresponding to the various lines were so mixed up by 

 superposition on the plate that little detail could be seen. The apparatus was 

 therefore modified in the following way : A three-prism Hilger spectroscope had 

 the collimator slit and the telescope removed, and the remaining parts were fixed on 

 a wooden stand in such a way that, while the light emerging from the last prism face 

 came out in a horizontal plane, the plane containing the prisms and the collimator 

 was inclined at 45 degrees to the horizontal. The spark, which took the place of the 

 removed slit, was turned round in a plane at right angles to the collimator axis until 

 it made an angle of 45 degrees with the plane of the spectroscope. The light 

 emerging from the prism face in a parallel beam fell upon the rotating mirror with 

 its axis vertical, from which it was reflected direct on to the camera lens. By this 

 arrangement a series of monochromatic images of the spark was produced en echelon 

 on the ground-glass screen of the camera when the mirror was stationary, the images 

 being vertical and the dispersion being inclined at an angle of 45 degrees to the 

 horizontal on the screen. The rotation of the mirror causes each of these images to 

 be drawn out in a horizontal direction ; and we can thus photograph simultaneously, 

 without much overlapping, a series of images of the streamers in the same spark 

 impressed on the plate by the monochromatic lights of its different metallic lines. 



