( !»;^ ) 



rised. From the above considerations it however appears that the 

 radiation might also be partially circularly polarised. We see at 

 the same time that, if in a given place of the spectrum the polari- 

 zation is righthanded, it must become lefthanded at the same place 

 by reversing the magnetization. 



§ 4. Arguments of the same kind may be used when the ob- 

 servations take place across the lines of force. Now, we place the 

 reflecting plane perpendicular to these lines. The magnetic 

 field remains unchanged ; consequently, the luminous motion must 

 have the same properties as its image. Hence the light, observed 

 in a given place of the spectrum, cannot be circularly nor eliipti- 

 cally polarised — neither completely nor partially. It must be either 

 unpolarised light, or plane polarised — wholly or in part — the 

 plane of polarization being parallel or perpendicular to the lines 

 of force. 



It needs scarcely be mentioned, that all observations are in agree- 

 ment with this conclusion. 



§ 5. A closer examination of the mechanism of radiation gives 

 us a relation between the light radiated along and the one radiated 

 across the lines of force. At least one conclusion concerning this 

 point lies at hand. 



Let M be a single molecule ot the source of light, and let three 

 rectangular axes OX, OY, OZ be drawn, the first along the line 

 of force. Let e be the electric charge in a point of the molecule, 

 having the coordinates x, y, z ; then we may call 2 ex, -^ e y, 

 — ez — ■ calculated for the entire molecule — the components of 

 the electric moment of the particle. 



These quantities are continually changing, and will perhaps be 

 extremely complicated functions of the time. By means of Fourikr's 

 theorem, we may however separate the parts that have a deter- 

 minate period T. We will confine ourselves to these parts and 

 denote them by 5)?;», 51?»/, '^Iz. 



If now the dimensions of the molecule are very small compared 

 with the wave-length, then, the observer being supposed at a distance 

 of a great many wave-lengths, it may be deduced from theory, that 

 in ail points of OY, liglit is produced merely by the variations of 

 5Jf,c and 9]i^, 9}f.(; producing vibrations along, and 9ïï« across the lines 

 of force. Similarly for points of OX and OZ. 



Suppose, that, when viewing across the lines of force, f. i. from 

 a point of OY, in a given place of the spectrum light is seen 



