458 Dr. E. Goldstein on the Influence of the 



succession, to which succeeds the image represented in fig. 

 116 when the exhaustion becomes sufficient. 



We obtain then at first an image of the kathode itself — a 

 cross having nearly the dimensions of the cylindrical projec- 

 tion of the kathode upon the wall. The arms of the cross in 

 the image grow smaller and become the central lines of a 

 well-defined square, that appears as a luminous background 

 round the cross. As the density decreases, the square be- 

 comes smaller and its luminosity increases. Its sides lengthen 

 beyond the angular points, and form points resting on the 

 sides of the square. The central cross disappears ; only the 

 intersection of its arms remains as the bright centre point of 

 the whole figure. 



The square now becomes smaller; and the points superposed 

 on the sides become narrower in the same proportion. This 

 takes place in consequence of the concave sides of the contour- 

 lines approaching each other, and passing over each other, 

 12 d and 12 e. As this displacement continues in the same 

 direction while the density of gas decreases, the pair of arcs 

 which previously intersected finally pass apart, and leave a 

 dark space between their convex sides : thus finally, at 

 great exhaustion, the dark cross already figured in fig. 11 b 

 results. 



The series of changes thus described is typical also for the 

 successive images given by crosses or stars with other numbers 

 of arms. 



"We obtain in each case at first an image closely resembling 

 the kathode, and of nearly the same dimensions. Observation 

 with three- and four-rayed stars (fig. 13 and fig. 14), shows — 

 what was not evident with the four-rayed star, nor generally 

 with regular figures with an even number of arms — that these 

 figures are reversed images of the kathode, formed by the rays 

 from the kathode crossing each other. Next, the background 

 round these figures becomes brighter, bounded by as many 

 sides as the kathode-star has rays. The rays of the star- 

 figure form the smaller radii of the polygon so formed. As 

 the exhaustion of the gas continues, this polygonal figure 

 decreases, its surface becoming brighter; and the inserted star 

 disappears, its bright centre point remaining visible the 

 longest. On each side of the polygon appears a point similar 

 to those in fig. 12 d. The concave sides of the bounding 

 arcs approach each other more the further the exhaustion 

 proceeds ; and at last, as the result of displacements exactly 

 similar to those of figs. 12/ and 12 g f there appear dark rays 

 between bright fields, which again correspond to the metallic 

 arms of the kathode. 



