338 Dr. J. Kerr on a new Relation between 



preserved. Two holes, about T V inch wide, are drilled into the 

 block from its opposite ends ; they lie exactly as in continuation 

 of each other, in a line parallel to the longest edges of the block, 

 and midway between opposite faces ; and they terminate in well- 

 rounded bottoms at the centre of the block, with a short extent 

 (a quarter inch or less) of clear glass between them. Two fine 

 pillars of glass rise from a stand on the table, distant an inch or 

 two from each other. The block is placed across the pillars (at 

 about a foot from the table), its plate-faces vertical, and the line 

 of borings horizontal ; and in this position it is tied securely to 

 the pillars by coils of silk thread. Two thick wires of copper, 

 sheathed in gutta percha, have their bared extremities inserted 

 in the borings, down to the ends. As a provision against the 

 strongest electric action applied in any case, these wires are 

 coated very deeply with lac or sealing-wax at their junction with 

 the glass, and an inch or more outwards. The whole surface of 

 pillars and block is well varnished with lac — except a small space 

 which is left clean upon each of the plate-faces, to allow distinct 

 vision through the centre of the block. 



When the dielectric has been thus prepared, its transparency 

 is all that can be desired. Objects bright or faint are seen horizon- 

 tally through the central parts of the plate (between and around 

 the ends of the borings) quite as well as through a fine window. 



2. The electricity is obtained from a RuhmkorfFs induction- 

 apparatus, which gives a spark of 20 to 25 centimetres. The 

 dielectric just described stands upon the table close to the induc- 

 torium. The outer ends of the wires from the dielectric are 

 screwed into the knobs of the secondary coil. From the same 

 knobs two wires are led to the other side of the instrument, and 

 are connected with two insulated metallic balls, which act as 

 spark-terminals. The distance of these balls, or the actual 

 spark-length in air, is under the observer's control at every 

 instant. 



The ends of the secondary coil are separated thus at one place 

 by so many inches of air, and at another place by a quarter inch 

 of glass. V/hen the primary circuit is closed in the usual way, 

 through the oscillating rheotome of the instrument, the air be- 

 tween the spark-terminals is broken by a sensibly incessant dis- 

 charge, while the glass between the induction-terminals is tra- 

 versed by a strong electric force. By simple increase or dimi- 

 nution of the distance between the spark-terminals, the intensity 

 of electric strain thus produced at the centre of the glass block 

 may be raised or lowered at once, and in any degree, as the ob- 

 server pleases. I may mention that on several occasions, when 

 the instrument was working at full power and the spark-termi- 

 nals were drawn 1\ or 8 inches apart, strong discharges burst 



