186 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
terminal, or to any independent source of electricity of sufficient magnitude, as by 
connecting it to earth. In such cases, therefore, the question whether the effect will 
assume the discharge or the repulsion form will be decided by the same circumstances 
that control it in the case of the connexion to-earth ; viz., the position of the tinfoil 
relatively to the positive terminal, and the nature of the discharge, or, as the latter 
may in all probability be expressed, its magnitude and degree of interruptedness. 
We therefore have, as has before been pointed out, a standard phenomenon at every 
point of a tube illuminated by a sensitive discharge, viz., the to-earth or relief-effect. 
We can get other effects connected with this, and to a great extent dependent upon it, 
by varying the completeness of the relief afforded. These effects form a continuous 
series from zero up to the complete relief-effect; and this series can easily be passed 
through continuously by some such process as that above described, in which two glass 
rods and a connecting wire were employed to form the relieving system. Hence, if at 
a point of the tube we obtain by any process an effect on the sensitive luminous column, 
we can at once ascertain whether it belongs to the category of relief-effects or not. If 
it does not, there must exist some foreign element other than the mere facility for 
granting electrical relief in the influences brought to bear upon the tube. We now 
proceed to examine these special or non-relief effects. 
V.— On the special or non-relief effects produced on the sensitive luminous discharge 
by connecting it with the air-sparJc terminal. 
In the present section we shall at first suppose the intermittence to be caused by 
an interruption in the circuit between the source of electricity and the positive 
terminal of the tube. For convenience we shall express this fact by calling the 
positive terminal the air-spark terminal. 
If we connect a wire with a piece of tinfoil placed upon the tube, and connect the 
wire with any independent conducting system, we shall obtain, as we have seen, more 
or less complete forms of the relief-effect. Both the wire and tinfoil will, in the 
majority of cases, repel the luminous column. But if the wire be connected with the 
positive terminal a sudden change takes place in the phenomenon. Instead of the 
luminous column being repelled by the wire, the course of the latter along the tube 
(supposing it partly to rest upon the tube) will be marked by a bright line of lumi¬ 
nosity on the inner surface of the glass as though it had attracted the luminous column 
instead of repelling it. And the effect of the presence of the tinfoil is changed in a no 
less remarkable manner. Instead of the former repulsion, a tongue of luminosity will 
be seen apparently starting from the actual inner surface of the glass under the tinfoil 
and stretching toward the negative terminal of the tube, while the luminous column 
on the positive side of the tinfoil is usually depressed or repelled, and is often well nigh 
severed in two. If the tinfoil be in the form of a ring round the tube the appearance 
of the phenomena is very striking. In many cases the luminous column extending 
