8l> Dr. J. Kerr's Electro-optic 



plate glass, f of an inch thick, 8 inches long, and originally, 

 for convenience in boring, about 4 inches wide. The first 

 step in the construction is made with steel drill and turning- 

 lathe. Two fine holes, about y 1 ^ of an inch wide, are drilled 

 right through the block, one parallel to its length, and the 

 other crossing the former at right angles in the centre of the 

 piece. Each of the borings is parallel to, and equidistant 

 from, the two plate-faces. 



The plate is now reduced to a more convenient width, one 

 inch of it (as it stands in the diagram) being ground away at 

 the top from end to end, and similarly one inch at the bottom, 

 except that a piece with sensibly square section is left project- 

 ing below the plate, round the vertical boring as axis. The 

 plate is also made to taper at each end, as in the diagram, 

 though this is not essential. 



Two other borings are now made, each through the plate, 

 at right angles to the plate-faces. One is a tunnel, concentric 

 with the plate, shaped as in the diagram, about an inch in 

 height, and f inch in width, and leaving a good margin of 

 polished plate-surface all round its mouths ; the other is a 

 slightly tapering hole through the projecting piece mentioned 

 above, into which is fitted afterwards a stopcock of glass, 

 which is easily worked by hand so as to open and close the 

 vertical boring. The sides of the tunnel are carefully finished ; 

 they are sensibly plane, and perpendicular to the long boring 

 and to the plate-faces. In these and the following operations, 

 the polish of the plate is preserved with care, for which pur- 

 pose the surfaces are permanently guarded by a shell of hard 

 varnish. 



The electric terminals within the tunnel are two balls of 

 brass, originally spherical, and a quarter inch in diameter. 

 Two thin shafts of brass pass from the ends of the block through 

 the long borings, and are screwed firmly into the balls. Round 

 the outer end of each of the shafts is a pierced plug or washer 

 of india-rubber ; outside of this is a perforated disk of brass, 

 of rather smaller diameter than the washer, and well rounded, 

 which moves freely along the shaft ; and outside of each disk 

 is a brass ball of ^ inch diameter, which screws onto the end 

 of the shaft. To provide for the insertion of conducting wires 

 into these outer balls, two fine holes are bored through each 

 of them, along diameters perpendicular to each other and to 

 the shaft. The outer balls are screwed along the shafts until 

 the washers are very strongly compressed. To prevent all 

 possibility of leakage at the junction of inner balls and block, 

 each of the long borings has been widened a little at the inner 



