O. N. Rood on the Electric Spark. 221 
its surface; when the sulphate of iron. developing solution was 
a it as usual, clear, sharp images of the spark were 
obtain 
n considered by photographers as sensitive to light, these un- 
ese it probable that the electrical discharge 
eye” condenser on a little stand, which to avoid too great heat 
Was arranged so that its surface was about half way between the 
lens and its focus, the bright spot on the stand covering an oval 
‘pace Z of an inch in diameter and 2 inches long. A plate 
phate of iron solution, which brought out an intense image of 
pot. 
.) The image of the positive spark falling on a plate merel 
Wet with ierece of cae has a sosiliad and definite form : * 
this form is really traced by light, we should expect that the light 
Would be able to act on another sensitive plate —_ directly 
under and in contact with the first. This was found to be the 
pse: a glass plate coated with sensitive collodion, on its removal 
from the nitrate bath, was covered with a piece of the very thin 
= used for microscopic purposes, the latter having previously 
Moistened by a solution of nitrate of silver ; sparks were 
then discharged on the covering plate of thin glass. When the 
. “T Was removed and the ilionion surface developed as usual, 
images of the spark were obtained. The definition was much 
impaired, and dis intensity lessened. When the thin glass was 
blackened So as not to transmit light, and the experiment re- 
no images were produced, not even by allowing a large 
number of sparks to strike the same spot. 
Apparatus for the production of the photographs.—For the gen- 
cation of sant she a al agile: machine was used: the 
eameter of the cylinder was seven inches, its length ten o 
Prime conductor exposed a surface of 200 square inches. 
