338 
MR. T. ROYDS ON THE CONSTITUTION OF THE ELECTRIC SPARK. 
now makes a sharp bend and immediately becomes practically vertical, growing more 
intense as it advances. We notice that this sudden bend at the commencement of the 
streamers becomes less pronounced as the order of the streamers increases, and that 
the inclination of the streamers is becoming less steep than the steep portions of the 
first three or four. 
[Added, April 3.—An enlarged drawing of the lead line X 4386 of fig. 5 is shown 
in the accompanying figure, the lines representing the first edges of the streamers. 
8 7 6 5 4 3 2 ' 
_ + _+ -+ -->• 
Drawing of A 4387 (Pb) of fig. 5, Plate 29. 
The instantaneous polarity of the electrodes is marked and the streamers from 
electrodes when positive are shown dotted. The first edges of the 4th and Gth 
positive streamers are not visible.] 
We have also to consider the second class of line, which comprises the short lines of 
the spectrum. An example is X 4561 of the bismuth spectrum shown in Plate 29, 
fig. 7. These lines have a very high velocity, for the first streamer is nearly vertical. 
Streamers proceed from the electrode both when positive and when negative, although 
stronger in the latter case. The commencement of the streamers is not bent as was 
noted in the first class of lines, and the first streamer is never overtaken by a following 
one. Although the measurements of velocity are most uncertain near the electrodes, 
yet this greater velocity is real and not due to uncertainty of measurement. On 
account of the width of the short lines becoming smaller at their tips in the stationary 
photograph, the readings have been taken on the most intense portion of their 
streamers ; any error due to this cannot be great, for the lines are almost as sharp as in 
the stationary photograph. 
6. Interpretation of the Envelope. —We now attempt the problem of finding the 
velocity of the metallic vapour. So long as the vapour first produced remains 
luminous it must indicate the velocity with which it is travelling, i.e., the first 
