O. N. Rood on the Electric Spark. 371 
interior being occupied by a canal with sharp edges, along which 
are disposed sharp round dots, &c. The portion of the spark 
not moved by the current of air consists of a mass of minute, 
rather ill-defined dots and rings, or simply of a uniformly 
shaded circular disc. Both of these latter forms seem identical 
with those produced by frictional electricity, and we are there- 
fore led by the inspection of the negative figure to the same con- 
clusions arrived at by the study of the positive. It often hap- 
pens that from the collection of dots and rings, two or three of 
the black tails have their origin, 
showing that a double or triple spark 
been generated; see fig. 22, 
magnified 4 diameters. It will be 
noticed that the actinic power resides 
mainly in the “atmosphere,” the col- 
lection of dots and rings being com- 
paratively faint. 
_ When the length of the discharge 
Is increased, the figure remains un- 
altered in character, except that the 
tail takes its origin from figures like 
12; see fig. 23 (only partly drawn), 
length of spark ,*, inch. 
tional electricity, often vanishes entirely, a result quite in ac- 
cordance with the theory of Prof. Rijke. 
On certain fine Markings which often accompany the Negative Figure. 
When the sensitive plate is used as positive electrode, the ne- 
gative figures, obtained without the use of a current of air, are 
often accompanied by peculiar fine markings, which usually ex- 
tend outward toward 24. 
ey  consequentl a : 
bear a much agter magnifying power than the negative figure 
itself An example of this kind is seen in fig. 24, magnit 
seven diameters, the distance of the electrode from the sensitive 
