MR. W. CROOKES ON RADIANT MATTER SPECTROSCOPY. 
723 
of the problem, the very last explanation likely to occur to the enquirer would be that 
these elusive shifting bands were due to the presence of two elements almost universally 
distributed, and that these two elements should be yttrium and samarium,—yttrium 
one of the rarest of known elements, and samarium, almost unknown at the time its 
spectrum reaction was first discerned. 
Description of the Coloured Lithograph. 
PLATE 75. 
The uppermost spectrum is that of yttric sulphate in the radiant matter tube. It 
is copied in colours from the author’s Bakerian Lecture already quoted, and is placed 
at the head of the coloured spectra for convenience of comparison. 
The other spectra are coloured copies of the illustrations in the text, and are 
designated by the same numbers. 
