Fah BBSneal 
XV. 
RADIATION PHENOMENA IN A STRONG MAGNETIO FIELD. 
By THOMAS PRESTON, M.A., F.R.U.I. 
(Prats XXT.) 
[Read Drcemprr 22, 1897. ] 
In March, 1897, the Philosophical Magazine contained a communication by 
Dr. Zeeman, in which he announced the important discovery that when a source 
of light is placed in a strong magnetic field, the spectral lines become sensibly 
modified in appearance. The modification then described by Dr. Zeeman, as 
recorded by eye-observations, consisted essentially in a broadening of the bright 
lines of the spectrum. Fortunately, the electromagnetic theory has been so far 
advanced as to deal with this phenomenon, and Prof. Lorenz at once pointed out 
that each broadened line should exhibit certain peculiarities of structure, namely, 
that it should really consist of three lines, superposed or separated, according to 
the strength of the magnetic field; that is, when the source of light is placed in 
the magnetic field and viewed across the lines of force, each spectral line should 
be divided into three, or become a triplet, and further, that each line of such a 
triplet should be plane polarised, the outside lines being polarised in one plane, 
while the middle line is polarised in a perpendicular plane. 
This polarisation was very easily tested, and the anticipations of theory 
were found to be in exact accordance with the experimental facts. Further, 
if the effect of the magnetic field is to resolve the lines of the spectrum in 
the manner here indicated, but still if the strength of the field is not sufficient 
to effect sufficiently wide resolution to completely separate the components 
of the triplet, but leaves them overlapping, so that, when viewed, the modified 
line does not appear as a triplet, but merely as a broadened line, then the 
constitution of this broadened line may be studied, to some extent, by 
means of the polarisation of its constituents. For, if a Nicol’s prism be 
placed in the path of the light, it will, in one position, cut off the central 
constituent, and leave the middle of the line dark or weaker; while in the 
TRANS. ROY. DUB. SOC., N.S. VOL, VI., PART XV. 3M 
