and the Rotatory Dispersion Formula. 411 



cannot be made out when they first appear. As the density 

 of the vapour increases, the components separate, four lines 

 being distinctly visible. This condition is shown in Plate V. 

 fig. 1 a. The lines are formed by the constituents of the 

 white light bordering the two absorption-lines, which having 

 suffered a rotation of 90° in the magnetic field, is enabled to 

 pass the second nicol. The lines continue to separate, 

 becoming broader, and presently we see between them a 

 second pair of lines, for which the rotation is 270, the two 

 dark regions between representing rotations of 180. This 

 stage is shown in fig. 1 b and c. Plate V. In fig. 1 b, the two 

 inner 90° lines are beginning to fuse together, the centre 

 being partially dark however. In fig. 1 c, the fusion is com- 

 plete and the centre of the system of lines is bright. With 

 a further increase in the vapour-density, the outer lines (90°) 

 separate still further, and widen out into broad flares of light, 

 and other lines appear between them, corresponding to 

 larger rotations, the system resembling a set of diffraction 

 fringes, as shown in fig. 2. The centre of the system, as I 



T%. 2. 



shall designate a point midway between 13 j and D 2 , becomes 

 bright and dark in succession, as is shown in the succeeding 

 figures of Plate V. fig. 1. Only a few of the inner lines show 

 in the photographs, as they are very close together, and the 

 vibration of the building prevented their resolution in the 

 photograph. They could be distinctly seen with the eye- 

 piece, and accurate settings of the cross-hair could be made. 

 Their position with respect to the D lines was very accurately 

 determined by means of a filar micrometer, settings being 



