452 Proceedings of the Royal Society of Edinburgh. [Sess. 
graphing farther into the visible spectrum and into the ultra-violet, or in 
making the more accurate measurements which are possible in spectra of 
higher order. 
It is proposed to explain here a new method which has been employed 
by the author for working in the higher orders without the necessity for 
taking so many photographs as were detailed above. The method depends 
on a study of the relative intensities of the components into which the 
spectral lines are resolved when the source of light is situated in a powerful 
magnetic field. 
Zeeman has shown * that in general a single line breaks up into a 
triplet when the light is examined in a direction at right angles to the lines 
of force of the magnetic field. Thus a line of oscillation-frequency n 
gives rise to three lines whose frequencies are n and nAz where e is 
47rm 
the charge of the electron, m its mass, and H the intensity of the magnetic 
field. This symmetrical triplet has each component line plane-polarised; 
the two outer components are polarised with their vibration directions 
perpendicular to the lines of magnetic force, and the central component is 
polarised parallel to the lines of force. According to the elementary theory 
of Lorentz, *]• if I v , I c , I r be the intensities of the components taken in order, 
beginning with the most refrangible, then I„ = |T = I r . When Zeeman 
effect observations are made with a diffraction grating, it is generally 
found that there is a greater or less departure from the above relationship. 
In some cases the three components may be of almost equal intensity, and 
in others the central component may be much more than twice as strong as 
the outer lines. Zeeman was the first to show that this apparent anomaly 
was produced by the selective action of the grating.^ When the plane of 
polarisation of the incident light is parallel to the rulings of the grating the 
intensity of the reflected light is a maximum, and when the plane is at 
right angles to the rulings the intensity is a minimum. Hence it is only 
when the planes of polarisation of all the three components are inclined at 
the same angle of 45° to the grating lines that the polarising effect of the 
grating is eliminated and the true relative intensities of the components are 
apparent. 
In many cases the action of the magnetic field is to produce more 
complicated types of resolution than the triplet. The elementary theory 
assumed a single electron, free from constraint, showing three degrees of 
* P. Zeeman, Phil. Mag., xliv. pp. 55 and 255 (1897). 
t H. A. Lorentz, Rapp. pres, au Congres Intern, de Phys., Paris , iii. p. 29 (1900). 
I P. Zeeman, Zittingsversl. Kon. Akad. v. Wetensch., Amsterdam, Oct. 1907. 
