ON RADIATION FROM A SOURCE OF LIGHT IN A MAGNETIC FIELD. 63 
Radiation from a Source of Inght in a Magnetic Iield.—Preliminary 
Report of the Committee, consisting of Professor GEORGE IRaNncis 
FirzGERaLD (Chairman), THomas PRESTON (Secretary), Professor 
A. Scuuster, Professor O. J. LopGE, Professor 8. P. THompson, 
Dr. GERALD Mo.toy, and Dr. W. E. ADENEY, 
Tue work undertaken by this Committee has not yet terminated. This 
occurs partly from the difficulties which arose in obtaining a satisfactory 
supply of electric current with which to excite the powerful electro-magnet 
now in the hands of the Committee, and partly from the circumstance 
that the Secretary was not always free to work at such times as the staft 
ot the Royal University found it convenient to permit research work in 
the Physical Laboratory. 
Considerable advance has been made, however, during the past session, 
and the magnetic perturbations of the spectral lines of several substances 
have been observed and photographed from one end tothe other of the 
spectrum. A considerable amount of work remains to be done in this direc- 
tion still, and this we hope to complete in the near future. 
The chief points of interest determined by the Committee since its 
appointment are as follows :— 
1. On Friday, September 9, 1898, Professor S. P. Thompson attracted 
the attention of the British Association (see ‘ Brit. Assoc. Report, 1898,’ 
p- 789) to an elegant experiment devised by Professor Righi for the 
purpose of illustrating the absorption of light in a magnetic field. This 
experiment was stated by Professor Righi to succeed only when the light 
traversed the field along the lines of force, but it appeared to us from 
theoretical considerations that similar absorption should also take place 
when the light traverses the field across the lines of force. On trying the 
experiment on the following Tuesday (September 13, 1898), it was found 
at once that the experiment was capable of demonstrating absorption 
across the lines of force! as markedly as that ascertained by Professor 
Righi along the lines of force. This result was also ascertained subse- 
quently, and independently, by M. Cotton.” 
2. The next point of interest consisted in placing beyond doubt that 
the various modified forms of triplet, that is the quartets, octets, c., are 
not produced by reversal or any other extraneous cause, but are true 
magnetic perturbations of the same kind as the normal triplet, which is to 
be expected from the simplest theoretical considerations. An account of 
the experiments by which this was determined will be found in the 
‘Philosophical Magazine’ for February 1899 (‘ Phil. Mag.’ vol. xlvii. 
p- 165). 
oe this inquiry it was found that in a very strong magnetic 
field the quartets ultimately became resolved into sextets, the side lines 
of the quartets splitting up into pairs and separating as the strength of 
the field gradually increased. 
These quartet forms, and various other types of perturbation, were 
observed by Mr. Preston in the beginning of November 1897, and were 
shown at the following meeting of the Dublin University Experimental 
1 See Nature, lix. 228-9, January 1899. 
* Comptes Rendus, 1898, 127, p. 953. 
