516 Scientific Proceedings, Royal Dublin Society. 
the screen, as shown in figure 2. The shadow of the central nail 
appeared as a black spot a little larger than the head of the nail, 
and the shadows of the other nails went out symmetrically from. 
the centre: those above went upwards, those below downwards; 
those on the right went to the right, those on the left went to the 
left. The interpretation of these facts was very simple and clear: 
Figure 1. 
the area of radiation was small, as shown by the sharpness of the 
shadows, and the centre of that area was in the line of the central 
nail produced. 
The question now was, what would happen to the shadows if I 
moved the arm B round the centre CO, thus making the board of 
nuls revolve in a circle to which it would always remain 
4 
