120 
Proceedings of the Royal Society of Edinburgh. [Sess. 
unsatisfactory for our purpose. With the Nernst lamp, for example, the 
intensity of the ultra-violet available proved to be only a very small 
fraction (less than a hundredth) of that obtained from our spark in 
hydrogen, and quite unsuitable for a research of this character. 
A quartz convex lens of about five inches focal length was attached to 
the same stand as the spark apparatus, so that the spark gap, T T, was at a 
distance from the lens equal to its focal length, thus giving a perfectly 
parallel beam of ultra-violet light. 
Apparatus and Method of Experiment. 
The photo-electric currents were measured from a piece of platinum 
foil to a copper disc placed some 1*2 centimetres in front of it : the 
apparatus used for the purpose is shown diagrammatically in fig. 2. 
One of us (Yarley, Phil. Trans., p. 439, 1903; note on p. 455) had 
previously found that for measurements at very low pressures, say below 
