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LIX. 
SHORT ACCOUNT OF AN EXPERIMENT TO DETERMINE 
THE EXACT POSITION IN A FOCUS TUBE FROM WHICH 
THE X-RAYS ARE EMITTED. By THE VERY REV. 
GERALD MOLLOY, D.D., D.Sc. 
[Read Drecemper 16, 1896. Received for Publication Maxcu 26 ; 
Published Jury 9, 1897.] 
I roox a deal board, 7 inches long by 5 broad, and three-quarters 
of an inch thick, into which I drove fifteen long slender nails, 
making three rows, with five nails in each row. The nails had 
small circular heads, and when fixed in position stood about one 
inch above the level of the board. This board I now attached to 
the back of a fluorescent screen E, by means of two elastic bands, as 
shown in figure 1 (p.516). The fluorescent screen was mounted on 
a stand A, the foot of which fitted tightly into a wooden socket F, at 
the end of an arm B, capable of moving round a centre C. The 
centre of motion C was simply a screw driven into the lecture 
table, and allowing the arm to move freely round in a circle. 
The Focus ‘Tube was now carefully adjusted so as to make the 
plane of the platinum plate vertical, with its centre in the hori- 
zontal line of the middle nail in the deal board, and in the vertical 
line passing through the centre of motion CO; and the fluorescent 
screen was made perpendicular to the arm B, so that in all 
positions of the arm, the screen and the deal board attached to 
it should be tangential to the circle described. 
When these arrangements were completed, I placed the wooden 
arm B in such a position that the plane of the fluorescent screen 
made an angle of about 45° with the plane of the platinum plate 
in the Focus Tube. The room having been then darkened, the 
current was ttirned on, and the shadows of the nails appeared on 
