A Fundamental Experiment in Electricity. 381 



thick, 183 cm. long, and 30 cm. wide. The two beams are 

 then received by the telescope-lens F (a Steinheil achromatic) 

 28*4 in. (72*1 cm.) focal length, and produce interference- 

 bands in the focal plane of a cylindrical eye-lens G, made of 



mm. in diameter. 



a well-worked glass rod about 4 



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I t V 



I 



' H F 



I /> A G 



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fit 



W/MSHUftST AtACHtM£ 



The brass tubes were half-cylinders, obtained by splitting 

 a complete tube about 5 cm. in diameter and 3 mm. thick into 

 two equal halves, by sawing it from end to end. The sawn 

 edges were then planed flat, and between the two halves was 

 placed the sheet of ebonite, along the middle of either side of 

 which was pasted a strip of tinfoil a little wider and a little 

 longer than the tubes. The edges of the tubes were pressed 

 firmly against the tinfoil, and the tubes were held in position 

 "by a series of stout insulating ebonite bars carried by a light 

 wooden frame, not shown in the diagram. In this way 

 the separating wall of ebonite was prevented from warping 

 and was kept pretty accurately plane, at any rate along its 

 middle part, which was necessary on account of the nearness 

 to each other of the two slits D D. 



From the middle of the outside of each tube projected a 

 brass rod with a knob, and these knobs could be connected 

 with a Wimshurst induction-machine (see fig. 2). When the 

 machine was worked a potential-difference was established 

 between the two tubes, and this increased until a spark passed 

 between adjustable knobs at K, which were kept about 1^ inches 

 apart (30 mm.). The optical apparatus at one end, and one end 

 of the frame carrying the tubes and partition, rested on a table 

 standing on a base of concrete, the other end of the frame and 

 the remainder of the optical parts on a similar table ; the 



Phil Mag. S. 6. Vol. 10. No. 57. Sept. 1905. 2 D 



