SECTION III., 1911. [3] Trans. R. S. C. 
PRESIDENTIAL ADDRESS GIVEN BY PROFESSOR J. C. 
McLENNAN BEFORE SECTION III OF THE ROYAL 
SOCIETY OF CANADA ON MAY 16TH, 1911. 
(ABSTRACT.) 
Professor McLennan after conveying his thanks to the Fellows for 
honouring him by electing him to the Presidency of the Section, opened 
his address by some remarks on the trend of modern physical research. 
Most extensive additions, he pointed out, had been made in recent 
years to our knowledge of the constitution of matter mainly through 
the investigation of problems dealing with the phenomena of radio- 
activity and of ionisation in gases by different types of radiation. 
He pointed out that considerable progress had been made, too, 
through the study of the spectra of the elements and through the 
behaviour of the lines constituting their spectra under a variety of 
physical conditions. 
In the treatment of this part of his address he gave a number of 
illustrations of the possibility of grouping the lines in the spectrum 
of an element into series in such a manner that for each series the 
frequencies of all the lines belonging to it could be represented by a 
single mathematical formula. The first formula of this kind was given 
by Balmer for the spectrum of hydrogen, but after him equations for 
the lines of other spectra had been established by anumber of other 
physicists including Rydberg, Schuster, Kayser and Runge, Ritz and 
Hicks. 
In the spectrum of sodium and the other alkali metals three series 
of double lines had been found which are distinguished by the names 
of principal series, first subordinate or diffuse series, and second sub- 
ordinate or sharp series. In the spectra of magnesium, calcium, stron- 
tium, zinc, cadmium, and mercury, three series of triplets had been 
identified. In the spectra of such elements as gold, copper and iron 
some isolated series had been discovered, but the majority of their 
lines had not yet been disentangled. 
Of all the series investigated those of the spectra of calcium, 
strontium and barium were among the most complete. 
In the spectra of these elements Saunders had been able to pick out 
(i) A principal series of main triplets. 
(ii) A first subordinate series of main triplets. 
(iii) A second subordinate series of main triplets. 
(iv) A principal series of doublets. 
