26 Mr. T. Preston. On the Modifications of the Spectra of Iron 



" On the Modifications of the Spectra of Iron and other 

 Substances radiating in a strong Magnetic Field." By 

 THOMAS PRESTON, M.A. Communicated by Professor GEO. 

 FRANCIS FITZGERALD, F.R.S. Received January 11, 

 Read January 20, 1898. 



Soon after Professor Zeeman announced his important discovery 

 that the spectral lines become sensibly'altered in appearance and con- 

 stitution when the source of light is placed in a strong magnetic field, 

 I determined to examine if the spectra of different substances are 

 equally or similarly affected, and also if the various lines in the 

 spectrum of a single substance are equally or differently affected by 

 the action of the magnetic field. The investigation of these matters 

 was undertaken also with the ulterior object of determining if the 

 phenomenon, for the discovery of which we have to thank Professor 

 Zeeman, could be made to afford any new information, concerning 

 the corresponding sets of lines in homologous spectra or in the 

 spectra of substances belonging to the same group of chemical 

 elements in fact, to determine if any new information could be 

 gleaned concerning those atomic or molecular vibrations which give 

 rise to the light emitted by incandescent matter, and thence to 

 approach more closely to a knowledge of the ultimate constitution of 

 atomic and molecular structures. 



For the purpose of this inquiry I availed myself gladly of the 

 opportunity afforded me of using the excellent Rowland's concave 

 grating mounted in the physical laboratory of the Royal University 

 of Ireland. This instrument is of the usual type, having a radius of 

 21'5 feet, and ruled with about 14,438 lines to the inch. It is fitted 

 with a camera box which takes a photographic plate 20 inches long and 

 2J inches broad, so that a length of the spectrum equal to 19 inches 

 can be photographed at a single exposure. As a consequence I 

 naturally decided to study the effect of the magnetic field on the 

 spectral lines by photography, rather than by eye observation, for the 

 latter, besides being applicable to the visible part alone, is more 

 liable to lend itself to the personal bias or the previously formed 

 opinions of the observer. The photographic plate, on the other 

 hand, does not lend itself to the imagination of the observer, but 

 gives a faithful record of the phenomena as they actually exist in 

 the image focussed upon it ; besides it enables one to compare the 

 effects produced on a large number of lines under identical circum- 

 stances, a point of great importance. 



In the case of a phenomenon which exhibits itself as a very small 

 effect, and one which it is difficult to obtain and observe, it is natural 



