A. W. Wright — Cathode Rays and their Effects. 237 



day a strong picture was made upon a photographic dry-plate 

 covered with a thin, white-wood board. In some of the earlier 

 experiments the objects to be tested were separated from the 

 sensitive plate by a screen of wood from a centimeter to a cen- 

 timeter and a half in thickness. Strong effects were produced 

 easily, but the pictures had a blurred appearance, which was 

 largely due to the increased distance from the plate, but in 

 part also to the diffusing; or scattering effect of the wood. 

 Later, an ordinary photographic plate-holder was used, the 

 objects being simply laid upon the ebonite slide, the vacuum- 

 tube being supported directly over it at a distance of from 

 four to six inches. In general it may be said that the 

 pictures have the appearance of shadows, and the conditions 

 which are necessary for the production of distinct shadows 

 by ordinary light apply to them, especially the greater dis- 

 tance of the radiating source, and nearness of the object 

 to the surface upon which it is to be projected. To secure 

 the latter condition the best arrangement was simply to wrap 

 the photographic plate in one or two thicknesses of black 

 opaque paper, as a protection from ordinary light, and to lay 

 the objects upon this. Increasing the distance of the vacuum- 

 tube improved the pictures in the matter of sharpness, fulness 

 of detail, and accuracy of form and dimensions, but the time 

 of exposure was correspondingly increased. In one experi- 

 ment a variety of objects was selected to test the action of 

 the rays upon different materials. On one side of the plate 

 were arranged four equilateral prisms, two of flint glass, one 

 of Faraday's heavy glass, and one of quartz. Two of these 

 were drawn out of line with the other two, so that a straight 

 steel wire laid along them was met by the sloping surfaces 

 from opposite sides. It was hoped that the trace of the wire 

 upon the plate would show the presence or absence of refrac- 

 tion, as it was so placed that the trace would be displaced in 

 opposite directions by alternate prisms in case of refraction. 

 Unfortunately the prisms proved too opaque to the rays to 

 show the desired effect. The quartz transmitted a little at the 

 thin edges, the flint glass prisms very little, and the heavy 

 glass still less. A block of calcite proved also to resist the 

 passage of the rays very strongly, and where some passed 

 through the thin edges, there was no trace of duplication of 

 the images, showing the absence of double refraction and polar- 

 ization. A metallic mirror set so as to throw a trail of rays 

 along the plate in case of regular reflection showed no such 

 effect whatever, nor did the polished surfaces of the prisms 

 and of the Iceland spar. The plate showed, however, that a 

 scattering of the rays occurred at various points where they 

 encountered the surfaces of the objects, and this effect was 

 noticed on many other occasions in the different experiments. 



