Mottoy—Source of X-Rays ina Focus Tube. 517 
tangential. This was a question which it was easy to answer,’ 
with the arrangements before me. When I moved the arm, the 
shadows remained absolutely fixed, and the figure on the screen 
remained, in all positions of the screen, exactly what it had been 
in the first position I had tried. It followed, that the centre of 
the area of radiation was in the line of the central nail produced, 
for all positions of the board; therefore it was at the point where 
all these lines would meet, that is, at or about the centre of the 
platinum plate. 
Figure 2. 
Having thus determined the position from which the X-Rays 
were emitted, I next proceeded to determine the size of the area 
of radiation, by means of a pin-hole image. I placed a metal 
plate, with a small round hole pierced in it, in a vertical position, 
facing the platinum plate, and about six inches distant from it ; 
and I placed the fluorescent screen at the same distance from 
the metal plate, on the other side. When the current was then 
turned on, as before, and the room darkened, I got a luminous spot 
on the screen, which, under the geometrical conditions of the 
1 Perhaps I should explain that the black patch close to the second nail from the 
right hand corner above, in the figure, is due to the presence of a piece of an old nail, 
which unknown to me was imbedded in the deal board. 
