384 Prof. Dove on the Electrical Light. 
brightest at last appears white. The colour of a source of light 
may, however, be investigated by allowing it to be absorbed by 
coloured dioptric media, or by investigating catoptric colours 
in its luminosity. I have availed myself of this method to com- 
pare the weaker electrical luminous phenomena with those of 
the spark. 
The electrical brush may be produced in two ways: by attach- 
ing the point either to the positive primary conductor itself, or 
to a second conductor into which sparks pass continuously from 
the primary conductor. In the former case its rays are closer, 
but less branched and diffused ; the brighter reddish-violet light, 
however, from which the rays are evolved is more intense, so 
that the whole brush appears to be illuminated by it. In the 
second case, the spark between the two conductors assumes, 
almost completely, the part of this bright basal point of the 
brush, the rays of which, however, are now much more branched. 
A similar difference is exhibited in the formation of the lumi- 
nosity in a large exhausted electrical egg. If the superior 
conductor passing through the stuffing-box be in immediate 
contact with the primary conductor, the reddish-violet, perpen- 
dicularly descending stream of light is intense, whilst the diffused 
light of the rest of the space is weak ; if, on the contrary, sparks 
be allowed to strike continually upon the superior conductor, the 
intensity of the perpendicular stream of light diminishes, whilst 
the whole space is filled with band-like whitish streaks of light, 
which incessantly change their form. This alone renders it 
probable that the perpendicular stream of light is the basal point 
of the brush which has become elongated in vacuo, and that the 
white bands correspond with its rays. 
If the brush be looked at through a deep blue cobalt glass of 
half an inch thick which effaces the middle of the spectrum, its 
ramifications are still seen very distinctly, whilst they disappear 
completely in a red glass. A green glass which so obscures 
the red that when they are superimposed in ordinary day- 
light one seems to have a board before one’s eyes, permits 
the passage of the rays, although more weakly than the cobalt 
glass. A blue picture on the red field appears to be brightly 
illuminated by the rays of the brush, upon a dark ground; a 
red picture on the blue field appears dark upon a bright ground, 
consequently just as when they are looked at in daylight through 
the deep blue glass. When looked at through an equilateral 
prism of Guinand’s flint-glass, in which I can see several of 
Fraunhofer’s lines of daylight with the naked eye, the rays of 
the brush appear nearly unchanged in colour, and only a little 
broader, whilst the bright basal point of the brush gives a spec- 
trum in which red, green, and violet appear brilliantly, and which 
