[MCLENNAN] PRESIDENTIAL ADDRESS 5 
of some of the lines—forexample the green line 5461 A°U—varied con- 
siderably with the source of the light. 
With the Cooper Hewitt lamp the line 5461 A°U was shewn to con- 
sist of a wide central main band together with three sharp and clearly 
defined satellites of greater wave length and three of a similar character 
of shorter wave length than the main line. With this source the intensi- 
ties of the satellites and the main line were shewn to follow roughly Max- 
well’s distribution law. 
When the Heraeus lamp was used and a strong current was passed 
through it the central broad band was shown to split up into two nar- 
rower ones. These lines and the four neighbouring satellites became 
equally spaced, and the intensities of the main lines and of all the satel- 
lites became approximately equal. 
Professor McLennan next proceeded to give an explanation of the 
Zeeman effect, an effect in which a modification is produced in the 
spectrum of an element when the source of the light is placed in a strong 
magnetic field. In this connection he exhibited a number of lantern 
slides which showed the effect with the mercury green line 5461 A°U. 
This line it was shown could be resolved with a strong magnetic field into 
a nonet of which the two pairs of outer triplets corresponded to vibra- 
tions perpendicular to the magnetic field while the inner triplets repre- 
sented vibrations along the lines of force. 
He then gave a short account of the steps he had taken to measure 
with an echelon grating the Zeeman effect of two of the satellites of the 
mercury green line, photographs were exhibited of his results which 
showed that it was possible to resolve these satellites into quartets whose 
inner doublets corresponded to vibrations parallel to the lines of force, 
and whose outer ones were due to vibrations perpendicular to the 
magnetic field. 
In order to obtain photographs of the magnetically resolved compo- 
nents of these satellites light of strong intensity and exposures of long 
duration were necessary. The first of these requisites was met by using 
a Cooper Hewitt lamp and the long exposures were made possible by 
placing this lamp in grooves bored through the pole pieces of the mag- 
net. With this arrangement the axis of the tube was parallel to the 
lines of force, and the effect of applying the magnetic field was to con- 
centrate the light along the axis of the tube. The glass of the tubes 
was thus protected during the exposure. 
