1 THE ROYAL SOCIETY OF CANADA 
(v) A first subordinate series of doublets. 
(vi) A second subordinate series of doublets. 
(vii) Three series of single lines. 
(viii) A group of lines consisting of 
(a) a triplet, 
(b) a doublet, 
(c) a single. 
(ix) A series of narrow triplets. 
One of the most remarkable things about the series lines of the 
elements was the fact that they could all be represented, whatever be 
the element to which they belonged, by a frequency formula, 
Ne 
IN Bes men 
2 
(m + a + EX 
m 
~ 
for example, in which m is given successive integral values 
and A,a and b are constants which vary with the series but in 
which N, appears as a constant practically the same for all the series | 
of all the elements. It was pointed out that the equality of the values 
of this constant occurring in the formule for the series of the different 
elements must be due to some corresponding equality in the properties 
of the ultimate particles of which these elements consist but that at 
present it was not possible to explain the nature of this similarity or 
to give a physical signification to the meaning of the length of time —. 
oO 
Before leaving the question of series spectra Professor McLennan 
exhibited some lantern slides and photographs of the spectral lines of 
mercury and shewed by means of these that he had been able, by using 
a suitable source for the light, to bring out a number of new lines and 
identify them as belonging to the principal and to the two subordinate 
series of triplets of this element. 
He then proceeded to explain the different methods which were 
followed in investigating the structure of narrow spectral lines. All 
these methods it was pointed out were based on the phenomenon of 
interference of light waves. By means of the Fabry and Perot system, 
by Lummer plates, and by means of an echelon grating, experimental 
illustrations were given of the analysis of some of the finest lines in the 
spectrum of mercury. 
The effect of a change in the source on the character of the light 
emitted was also discussed and by means of a Cooper Hewitt and a 
Heraeus quartz glass mercury are lamp it was shown that the structure 
