PEOFESSOE STOKES ON THE LONG- SPECTEIJM OF ELECTEIC LIOHT. 615 
Advantage of Broad Electrodes. 
At first I employed by preference wires or sharp pieces of metal for electrodes, in con- 
sequence of the greater facility with which the discharge passed, and the larger quantity 
of light given out by the spark. Certain considerations, however, led me to try broad 
electrodes ; and I accordingly procured electrodes of the common metals shaped like 
small watch-glasses, about an inch in diameter. These showed in some cases a most 
marked superiority over thin wires, exhibiting the invisible metallic lines in far greater 
strength, while with some metals there was not much difference. With copper, for 
example, the superiority was very great, with iron it was comparatively small. 
Instead of electrodes of this shape, it is sufficient to take two pieces of thick foil, make 
them slightly cylindrical by means of a round ruler, or a pencil, and mount them with 
their convexities opposed and the axes of the cylinders crossed. 
Besides copper, silver, tin, and aluminium show a great advantage of flat electrodes, 
and lead a moderate advantage, while with zinc, as with iron, sharp electrodes are nearly 
as good. Brass agrees in this respect with zinc, and not with copper, though it shows 
the copper lines very strongly. 
With such electrodes, however, the spark dances about ; and its unsteadiness is 
objectionable in some experiments. A good part of the advantage of flat electrodes is 
however retained if one only be flat, especially if this be negative, and the spark is now 
steadier. Instead of using the end of a wire to combine with a flat electrode, it seems 
rather better, according to a plan suggested to me by Dr. Miller, to bend a wire to a 
gentle curve lying in a vertical plane passing through the prism ; or the edge of a flat 
piece of metal may be similarly employed. 
On forming an image of the spark between a sharp and a flat electrode of copper, and 
receiving it on a fluorescent screen, the flat electrode gave the brighter of the two images 
already mentioned, and that, whether the electrode were positive or negative. 
On similarly forming an image of the spark between two very broad eloctrodes, and 
focusing for the rays of highest refrangibility, the image did not, as usual, consist of two 
separate dots ; but whether it was, that, from the shortness of the spark, the two ran into 
one, or that the rays belonging to the metallic lines of high refrangibility were emitted 
throughout the whole length of the spark, I am not quite certain ; but I incline to the 
latter opinion, as a separation of the discharge into two portions, corresponding to the 
immediate neighbourhood of the two electrodes respectively, could hardly have escaped 
detection had it existed. 
Arc Discharge., and Lines of Blue Negative Light. 
On diminishing the distance between the electrodes, formed suppose of copper wires, 
the brightness of the metallic lines at first improves, and afterwards changes but little, 
or, if anything, rather falls otf. On still further diminishing the distance, so that the 
electrodes almost touch, and the discharge passes with little noise, a new set of strong 
lines make their appearance in the invisible region of moderate refrangibility. In this 
MDCCCLXir. 4 P 
