204 Mr. J. E. H. Gordon on Dr. Kerr's Experiments 



The breadth of the black bands was about J that of the 

 bright ones. 



To test the sensitiveness of the apparatus, a black band was 

 brought upon the cross-wires, and a piece of glass | inch thick 

 and 2 inches square was compressed in the fingers. A very 

 moderate compression served to displace the black band, so 

 that its edge, instead of its centre, came upon the cross-wires, 

 while moderate compression in a small iron screw clamp moved 

 the band, so that instead of the centre of the black band, the 

 centre of the bright band next on the right or left, according 

 as the compression was vertical or horizontal, came on the 

 cross-wire. This corresponds to an apparent displacement of 

 |- inch, or a retardation of about \ a wave-length. 



The electrical arrangements. 



The coil. — The electricity was produced by a powerful in- 

 duction-coil by Apps, giving a thick spark 16 inches long in 

 air. It was provided with a vibrator and a clock contact- 

 breaker, both of which were used in the experiments. 



The coil was worked by 9 or 10 quart Grove cells. It was 

 managed by an assistant, while fine wires suspended by silk 

 threads carried the secondary current to the optical apparatus. 



The coil was placed behind the observer ; so that his eyes 

 were not dazzled by the sparks. 



The experiments. 



A piece of plate glass 1 inch thick and 6 inches square, had 

 holes drilled, as described by Dr. Kerr, in opposite edges, 

 the bottoms of which holes came within 1 J inch of each other. 



Quill glass tubes a foot long were put into these holes nearly 

 to the bottom. Inside the quill tubes were stout brass wires, 

 which reached to the bottoms of the holes. 



One tube and wire having been put in and the whole ap- 

 paratus warmed, it was set on one edge on the table, so that 

 the glass tube stood vertical. A kind of cell was made, consist- 

 ing of two panes of window-glass pressed against the faces of 

 the plate, and projecting above it, and two blocks of cork, the 

 thickness of the plate, between them above the plate. The 

 thin glasses were slightly greased, and the interstices between 

 them and the corks filled up with a little flour-and-water 

 dough. This then formed a cell 2 inches long, 1^ deep, and 

 1 inch wide. In the centre was the vertical tube. The cell 

 was now filled with Apps's electrical cement. This is a com- 

 pound consisting of shellac, &c, used for perforating glass by 

 means of induction-coils. It is so good an insulator that 

 blocks of glass whose diameter is not more than twice their 



