480 Mr Filon, On the variation with the wave-length 
The scale s, which consisted of a network of squares of 5 mm. 
side, and carried besides one horizontal and one vertical millimetre 
Oat 
v 
Fig. 2. 
scale, was ruled for me on glass with a dividing engine by Mr Edwin 
Wilson, of Mill Lane, Cambridge. It was viewed through a 
telescope U , in front of which was placed an analysing Nicol N. 
The slit T could be illuminated, either by a powerful arc lamp, 
or by a spark between metallic poles in air. An alloy of Lead, 
Tin and Cadmium was employed for this purpose. The use of 
this alloy was suggested to me by Mr H. C. Ramage, of St John’s 
College, to whom I am also indebted for finding out for me the 
wave-lengths of a large number of characteristic lines in the spark 
spectrum of this alloy. This spectrum has a large number of 
bright lines fairly uniformly distributed from the red to the 
extreme violet and is accordingly very useful for comparison pur- 
poses. In the diagram (Fig. 2) the spark holder is in position 
