LEWIS, ON THE ELECTRIC LIGHT. 17 
These and other considerations rendered it very desirable 
that some contrivance should be adopted by which this un- 
steadiness might be obviated, and the observer enabled at the 
same time to see the precise effect of every individual spark. 
This was accomplished by performing the experiments under 
the microscope itself, the mode of doing so being as follows :— 
Two plates of glass were cut so as to be about half an inch 
longer than the stage, and a small hole, about one eighth 
of an inch in diameter, was drilled in the centre of one of 
them. A copper wire, having one end finely pointed and 
turned up at right angles, was then placed between the glass 
plates in such a position that the turned end occupied the 
centre of the hole, but did not project above the surface ; the 
plates were then cemented together with marine glue, thus 
forming at the same’time an insulating stage and a holder for 
one of the terminal wires. ‘The end of a glass-dipping tube, 
mounted in the same way asa pair of stage forceps, served 
to hold and insulate the other wire, the finely pointed end of 
which was bent so as to enable it to be brought into the 
centre of the field in a straight line with the end of the wire 
in the glass stage, whilst, by the universal motion of its 
mounting, the length of the sparks could be easily regulated. 
A simple battery after the French pattern, a small induction- 
coil capable of giving three quarter inch sparks if required, 
and a rheotrope by which the current could be instantly 
broken or reversed at will, completed the apparatus, and 
rendered it possible to conduct the experiments with perfect 
ease and steadiness. The position of the poits having been 
carefully adjusted, the paper to be operated upon was placed 
upon the glass stage, illuminated both by’ transmitted and 
reflected light, and properly focussed, and ‘on. making the 
circuit the effect of every spark could be’ séen in a perfect 
and most beautiful manner. 
It is perhaps unnecessary to state that great care is required 
in thus dealing with so energetic 'an agent as electricity, 
which is ever on the alert for opportunities of completing its 
circuit by the shortest course through the best conductors, 
and which makes no excuses for inadvertence. If the wires 
are perfectly insulated from the stage, there need be no fear 
of sparks passing from the eye-pieces to the operator’s face ; 
but accidents are very liable to happen during manipulation, 
from the hands coming in contact with portions of the appa- 
ratus, whilst the eyes are attentively engaged at the binocular 
and the attention is absorbed by the increasingly interesting 
character of the observations. 
In repeating the foregoing experiments upon the stage of 
VOL. VII.—NEW SER. B 
