The spectrum B of the tube containing radium emanation is the one which we are comparing with the other two. When the radium emanation was first enclosed there was only a small quantity of helium developed in it, but after keeping for a year the quantity has greatly increased. After five minutes “sparking” (passage of the electric spark through the tube) the chief lines of helium become evident but faint in intensity. The present photograph B was obtained after forty minutes sparking, and one result of that longer “sparking” has been that a minute quantity of water vapour in the tube has been broken up--_so as to yield the hydrogen spectrum_, which is accordingly seen accompanying the now strong and brightly developed helium spectrum. The lines of the spectrum B which correspond with those of hydrogen are at once recognised by the juxtaposition (below) of the pure Hydrogen spectrum from another tube--C: the lines in B belonging to and indicating helium are also recognised by comparison with the pure helium spectrum of the tube A juxta-posed above. A very few of the lines in B must be due to other minimal impurities as they are not present either in A or C. Thirteen lines of the helium spectrum are thus photographed and recognised in the radium emanation. The following lines are present in the photographic but invisible spectrum of radium (not given in fig. 10), viz. at 381·47 µµ (the strongest line in the radium spectrum) and at 364·96 (a strong line). In the photographic but invisible spectrum of helium there are three very faint lines between wave-length 447·2 and 443·7 (appearing as two only in our photograph); a moderately strong one at 438·8; others at 414·4, at 412·1, at 402·6, and 396·5; a very strong one is present at 388·9, and a very faint one at 381·9. All these are seen in the photograph A and also in B. Special treatment and spectroscopes reveal four other very faint lines in the helium spectrum--the one furthest in the invisible direction (that is of highest refrangibility and lowest wave-length) being placed at 318·6 (Soddy).]