Rays of Positive Electricity. 227 



ground so that the tap can be rotated without injury to the 

 vacuum in the discharge-tube. The box containing the plate 

 was placed in front of a wi lie mite screen, S, fastened to the 

 end of the tube ; when the lid of the box was down the 

 positive rays could hit the screen ; the appearance on the 

 screen showed when the pressure in the tube was such as to 

 give well-developed positive rays and thus indicated when a 

 photograph could be taken with advantage. When this stage 

 was reached the coil or influence machine used to produce the 

 discharge was stopped, the lid of the box lifted by turning 

 the tap, the tap was turned until the lid came against stops 

 placed inside the tube, so that the plate was always in the 

 same position. The tube was surrounded by black velvet so 

 as to prevent light from outside reaching the tube, while the 

 light from the discharge and the phosphorescence of the 

 walls of the tube was prevented from reaching the plate by 

 tightly-fitting stops placed in the neck of the tube. The only 

 way that light could reach the plate was through the long 

 and narrow tube through which the positive rays themselves 

 passed, and this light would produce a small circular patch 

 on the plate coinciding in position with that produced by the 

 positive rays themselves when they were not deflected by 

 electric or magnetic forces. 



After the plate had been exposed to the rays for a suitable 

 time the coil was stopped and the tap turned so as to put the 

 plate back again into the box. The lid of the tube, D, which 

 was fastened on with sealing-wax, was taken off, the box 

 taken out of the tube, and the plate developed. The size of 

 the plate was 4 cm. x 4 cm. 



Many photographs have also been taken with an arrange- 

 ment devised by Mr. F. W. Aston, of Trinity College, shown 

 in fig. 2 (p. 228). In this method the plate is suspended by a 

 silk thread wound round a tap which works in a ground- 

 glass joint ; by turning the tap the silk can be rolled or 

 unrolled and the plate lifted up or down. The plate slides 

 in a vertical box of thin metal, light-tight except for the 

 opening A, which comes at that part of the tube through 

 which the positive rays pass ; the openings, which are on 

 both sides of the box, are circular and 5 cm. in diameter. 

 When the silk is wound up, the strip of photographic plate 

 in the box is above the opening, so that there is a free way 

 for the positive rays to pass through the opening and fall on 

 a willemite screen placed behind it, so that the state of the 

 tube with respect to the production of positive rays can easily 

 be ascertained. The box is large enough to hold a film long 

 enough for three photographs ; by lowering the plate until 



Q2 



