72 DK. S. E. MILNER ON THE NATURE OF 



the electrodes to the centre of the gap. The exception which has been taken to this 

 view has arisen in part from the difficulty of observing the Doppler effect on the 

 metallic lines which should be a concomitant of the diffusion of the vapour from the 

 poles,* and in part from the extraordinary results which the authors themselves 

 obtained in some metals for the velocity of the diffusion corresponding to the different 

 lines. In the case of bismuth and, in a less degree, of cadmium the different metallic 

 lines could be divided into groups of different curvatures which indicated different 

 velocities of diffusion towards the centre of the gap. As regards the former matter, 

 there does not seem to be involved any real difficulty to the explanation, as 

 Dr. SCHUSTER has himself recently shown, f The curious effect of the different 

 curvatures of the lines of the same element has, however, always remained more or 

 less of a difficulty in the way of a complete acceptance of their view. SCHUSTER and 

 HEMSALECH themselves refer to the possibility in the case of bismuth that the metal 

 may be a compound, and that the two kinds of molecules give rise to the differently 

 curved lines. Other explanations* have been made by different writers, but it cannot 

 be said that any explanation adequately supported by experiment has been forth- 

 coming. In view of this incompleteness in our knowledge of the constitution of the 

 streamers it seemed to me that further observations with a rotating mirror would 

 possibly be of value, and the investigations recorded below succeed, I think, in 

 throwing a clearer light on the nature of the streamers, and on certain other 

 phenomena which are characteristic of the spark. 



In the first experiments which were made the light of the spark was not submitted 

 to spectral analysis, but was observed as simply drawn out by the rotating mirror. 

 While they were only of a preliminary nature they nevertheless gave some interesting 

 results which merit a short description. The sparks were obtained by the discharge 

 of a battery of 12 Ley den jars which were charged by an induction coil worked by a 

 mercury platinum break. By means of a length of insulated copper wire, wound on a 

 cylindrical rod and containing terminals along its length, varying amounts of 

 inductance could be inserted in the discharging circuit. The poles between which the 

 spark took place were pointed metal rods, held in corks and placed, one vertically 

 above the other, in a small box with a single opening which cut off all light from the 

 r^)om except that by which the spark was examined. At its focal distance away from 

 the spark was placed a large lens throwing the light in a parallel beam on to the 

 rotating mirror, by which it was reflected on to the lens of an ordinary half-plate 

 camera. The mirror was fixed on the axle of a gear-wheel arrangement, and could be 

 rotated about a vertical axis by a motor at speeds up to about 150 rotations per second. 

 The induction coil gave one or two sparks every second, of which perhaps one in every 

 dozen was reflected into the camera. By looking through the back of the plate in the 



* HULL, ' Astrophysical Journal,' vol. 25, p. 1 (1907). 



t ' Astrophysical Journal,' vol. 25, p. 277 (1907). 



J Of. J. J. THOMSON, ' Conduction of Electricity through Gases,' p. 397 ; p. 520 in 2nd edition (1906). 



