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Proceedings of the Royal Society of Edinburgh. [Sess. 
were reflected into the instrument, it would increase the value of F by 
one-tenth. Also some of the exciting rays might arrive at S through 
being scattered on entering the glass or through not being parallel to the 
Diagram 2. — Uranium Glass. First specimen. 
direction of the arrow. Hence T + F — C would be positive and propor- 
tional to F. The nature of this error is shown in an exaggerated form by 
the following table, which was obtained by replacing the black paper on 
S by one somewhat greyish : — 
