Images in the Field of a Hittorfs (Crookes') Tube. 311 



red, as usual, the image of the opening is black. On raising 

 this object to a height of one centimetre above the screen by 

 a metal stalk cemented by wax to a glass disk (fig. 2) , we 

 obtain the image fig. 3. The white parts are the neutral 



Fiff. 2. 



Fig. 3. 



portions ; they are black in the image and correspond to the 

 solid parts of the object which were not in contact with the 

 screen. The image of the disk, which was in contact with 

 the screen, is red, which is indicated in the figure by parallel 

 lines. The image of the opening is markedly larger than the 

 opening itself and of an intense yellow colour. The rest of 

 the plate is also yellow ; this colour is represented in the 

 figure by a network of lines crossing one another at right 

 angles. The rectilinear margins of the object show them- 

 selves in the image as curved lines : in the image of the 

 exterior rectangle these curves turn their convexity inwards ; 

 while in the image of the interior rectangle the curves de- 

 scribed have their convexity directed outwards. The fact 

 that the image of the opening is markedly larger than the 

 opening itself shows that we could construct the image by 

 imagining curvilinear rays which, on passing the aperture, 

 converged towards certain points of the tube. 



(5) A rectangle cut from a sheet of lead with a rectangular 

 opening in the middle was placed beneath the ebonite plate 

 and in contact with it. The image of the solid parts is intense 

 yellow ; the image of the opening is black ; the rectilinear 

 margins of the opening are replaced by curves which are 

 convex inwards. The same object (fig. 4) was lowered one 

 centimetre beneath the screen. The image is represented by 

 fig. 5. The solid parts of the object (white in the figure) 

 are black in the image. Tbe image of the opening is red, 

 smaller than the opening, and bounded by four curves which 

 turn their convex sides to the centre of t\w figure. AVe 



