1899.] on Magnetic Perturbations of the Spectral Lines. 155 



and interesting conclusion, viz. that the constituent lines of this 

 triplet must be each plane-polarised — the central line of the triplet 

 being polarised in one plane, while the side lines are polarised in a 

 perpendicular plane. In fact the vibrations of the light forming the 

 central line are parallel to the lines of magnetic force, while the 

 vibrations in the side lines are perpendicular to the lines of force. 

 This prediction of tripling and polarisation from theoretical con- 

 siderations may be regarded as the key to the subsequent advance 

 that has been made in the investigation of this region of physics. 

 In order to understand it, let us place ourselves in Dr. Zeoman's 

 position when he found that the spectral lines became broadened by 

 the magnetic field, and let us be informed that this broadening is in 

 all probability a tripling of the lines accompanied by plane-polarisa- 

 tion. The question now is, " How are we to determine if this is the 

 case ? " 



It is clear that if the broadened line is really a triplet, then the 

 components of this triplet must be so close together that they over- 

 lap each other, and so appear to the eye merely as one broad line, as 

 illustrated by the model which is here before you. [Model illus- 

 trating the overlapping shown here.] We know that the spectral 

 lines are not infinitely narrow lines, but are really narrow bands of 

 light of finite width, and consequently we are quite prepared to 

 regard the magnetically broadened line as an overlapping triplet ; but 

 we cannot remain satisfied until we have proved beyond all doubt 

 that it really is a triplet, and not merely a single broad line. To 

 do this, Dr. Zeeman made use of the second prediction of the theory 

 —namely, that the constituents of the triplet must be plane-polarised. 

 If this is so, then the outer edges of the broadened line must be plane- 

 polarised, and therefore by introducing a Nicol's prism into the path 

 of the light it must be possible to turn the Nicol so that the plane- 

 polarised edges shall be cut off, and the breadth of the line shall be 

 reduced to its normal amount. In fact, in this position of the Nicol 

 the outside lines of the triplet are extinguished, and the central 

 component alone remains. This component is, of course, the same 

 in width as the original line, and consequently when the outer 

 members of the triplet are extinguished all the magnetic broadening 

 of the line is removed. When the Nicol is turned through a right 

 angle the central component of the triplet is extinguished, while the 

 side lines remain ; and, if these side lines are sufficiently separated 

 so that they do not overlap, then, when the central line is removed, 

 a narrow dark space will exist between the side components, which 

 represents the space intervening between the outer members of the 

 triplet, as illustrated by this photograph. [Slide shown.] 



But even though we may be able to so increase the strength of 

 the magnetic field that when the central component of the triplet is 

 removed by a Nicol the side lines stand apart with a clearly defined 

 interval between them, yet this in itself does not absolutely satisfy 

 us that the broadened line is a triplet. It might be contended that 



