82 DR. S. K. MILNER ON THE NATURE OF 



distance from the pole towards the centre of the spark it is too low by itself to affect 

 the plate. A short distance to one side of this point the second streamer (itself also 

 insufficient to affect the plate) is superposed on it, and the two together will have 

 sufficient intensity to produce a photographic effect. The apparent outline of the 

 luminous vapour will be thus displaced to one side. A little further out, where the 

 individual streamers are still less bright, it will require the combined effect of three 

 streamers to affect the plate, and the apparent limit of the vapour will be displaced 

 still further to the side of the true edge of the first streamer. Thus the apparent 

 velocity of the vapour towards the centre of the spark will over a certain region 

 always be less than the actual velocity, and the extent to which it is so will depend on 

 several circumstances, of which the rate at which the luminosity of the streamers dies 

 away and the period of the oscillations of the spark are two of the chief ones. The 

 period of the oscillations is the same for all the lines in the same spark, but the rate 

 at which the intensity of the light of the streamers corresponding to the various lines 

 dies away is, as we have seen, very different, hence the apparent velocities due to this 

 cause would also be different for the different lines. It is necessary, of course, that 

 the period of the oscillations should be sufficiently large for the streamers to be 

 separated from each other to an appreciable extent, if there is to be produced a 

 measurable change in the velocity by means of this effect. That this is the case in 

 the measurements of Messrs. SCHUSTER and HEMSALECH is, I think, evident from 

 an inspection of their figure 2G of the bismuth spark, where the streamers in the 

 lines of short duration are clearly separated in the photograph. Hence it seems 

 to me that the effects of varying velocities for the different lines which they 

 obtained in their researches may be adequately accounted for in the way described 

 above. 



Another point to which attention was paid in the examination of the photographs 

 was the possible existence of differences between the anode and the cathode streamers 

 in each oscillation. It would be interesting, as bearing on their constitution, if any 

 lines were found which were associated with either the anode or the cathode discharge 

 to a greater extent than other lines, but a close scrutiny has not revealed the 

 existence of any such cases. The only case of any difference in the character of the 

 anode and cathode streamers which I have found is that shown in the mercury spark 

 of fig. 29, and the difference here is one which affects all the lines of the metal 

 equally. The cathode streamers are conical in shape and taper to a point, the front 

 edge of the streamer having an inclination which corresponds to a velocity of 

 530 metres per second, and the back edge to 1100 metres per second. The 

 positive streamer is weaker than the negative one and has its two edges parallel, 

 the inclination of each representing a velocity of 1100 metres per second. The 

 effect is very striking in the negative, and it is not easy to explain it in terms 

 of under-exposure, or, indeed, in any way which at present would not be purely 

 speculative. 



