156 0O.N. Rood on the discharge of a Leyden jar. 
ployed together. The arrangement is seen in figurel. Th 
light diverging from 8, was received by an achromatic lens L, 
of nine inches focal length ; it then fell on the plane mirror at 
M, and was finally brought to a focus on the plain or ground 
glass at G, which was distant from the mirror about 16 inches. 
For the purpose of measuring the extent to which the image a 
of the spark had been drawn out by the motion of the mirror, 
a scale consisting of bright lines on a dark ground, was pro- 
jected on the ground glass, as seen in the figure at CC, the _ 
flame of a small lamp furnishing the illumination. The spark — 
generated between the balls or wires of a spark-microme- ~ 
ter, graduated in millimeters ; this was placed of course in one — 
of the conjugate foci of the achromatic lens, An automatic — 
arrangement for breaking the circuit from three to six times 
‘in a second, was employed, mercury and alcohol being used 
in it. The coil was a large one by Ritchie of Boston ; it was 
excited during these experiments by two cups of the Bunsen 
form ; the simple sparks furnished by it did not at the time 
exceed seven inches in length. The Leyden jar had a coat- 
ing of 114-4 square inches, and all the connections were made 
as short as possible, so as to offer the least amount of resis- 
tance to the flow of the electricity. The experiments were — 
usually made in a room not entirely darkened, as in a perfectly — 
dark room it was found difficult to preserve the proper “accom- — 
modation” of the eyes. : 
If no means are employed to late the moment at which — 
the discharge takes place, it is evident that images of the spark 
