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them to be easily cemented against the faces of the partition. The latter was 

 circular, its diameter being equal to the outside diameter of the vessel. A narrow 

 strip of brass, 2 millims. in thickness, was soldered to each face, extending all round 

 the circumference, except for a gap of 4 '5 centims. at the top. When the ground 

 edges of the two halves of the vessel were cemented against these strips there was 

 left at the top a slit on each side of the brass plate, 4 '5 ceutims. long and 2*5 millims. 

 wide. The double slit was closed by covering it with a strip of thin aluminium 

 cemented to the outer surface of the glass and to the edge of the brass partition. A 

 thin layer of air in contact with each surface could thus be exposed to the Rontgen 

 rays from a source placed vertically above the partition. 



Fig 2. 



Each half of the apparatus contained a second brass plate fixed parallel to the 

 central plate at a distance of 1 '8 centim. from it. These had the form of equilateral 

 triangles, and were fixed to the glass with sealing wax at the corners. There was 

 room between the sides of the triangle and the walls of the vessel for the air to 

 escape from between the plates at the moment of expansion. 



The metal plates were, as in the former apparatus, covered with wet filter paper. 



The anticathode of the focus tube, which generated the rays, was fixed by eye 

 vertically above the central plate of the apparatus. A lead screen connected to 

 earth and provided with a slit, 4 millims. in width, was placed about 2 centims. above 

 the aluminium window of the cloud vessel. The final adjustments were made by 

 moving the screen until, when both side plates were kept at the same potential 

 (higher than that of the central plate, which was always earthed), exactly equal fogs 

 were obtained on the two sides, with expansions sufficient to catch the negative ions. 



To make the fogs on the two sides readily visible simultaneously, a horizontal stratum 

 of the air in both halves was illuminated by the light from a horizontally placed 

 luminous gas flame brought to a focus within the apparatus, the source being behind 



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