(841 ) 



very narrow bands at — 193^ in tlie violet and blue; in the orange 

 some heavy broad bands, moreover a strong band 599,0 — 599,3, a 

 pretty faint band 600,9—601,4. The plate was now investigated 

 with the median line (dividing the acute angle formed by the axes 

 into two equal parts) vertical in a iield of 40 kgs. 



With vertically polarized light band 599,0 — 599,3 appeared to be 

 subject to a distinct widening of 0,1 mi ; the other band also became 

 wider and vaguer. With horizontally polarized light the phenomenon 

 was analogous, but less distinctly to be seen ; on the other hand 

 some of the wider bands then show an unmistakable widening ^). 



JSfeodymium sulphate [Nd^{SO^)^. SH^O]. 



§ 23. As a supplement to what was communicated in § 19 a 

 number of plates of different thickness were more fully examined ; 

 they again contained both optical axes; the line dividing the acute 

 angle was again placed in a vertical position. 



Group of bands in the blue at — 193° ; 8 of these bands were 

 measured. For the sake of brevity we have been obliged to draw 

 up the results in a table, where A denotes the wave-length, /?„ the 

 width with field off, /i, the width in a field of x kgs., d^ the wide- 

 ning; in case a multiplet is formed, the distance of the centre-lines 

 of the extreme components is denoted by dl; the value of dljX^ is 

 expressed in cm—', as is now usually done. 



41 Kilogauss. — Plane of polarisation horizontal — Thickness 0.3 mm. 



1) From a copy that Prof. Kamerlingh Onnes kindly sent us of the paper by 

 himself and Mr. J. Begquerel (These Proc. X, p. 592) we now infer that the 

 results given for the silicates of Pr and Nd really apply to the sulphates; we 

 had then nearly finished our observations ; as, moreover, these were made at 

 — 193° instead of — 253' and in a much stronger field, the two series of results 

 are not directly comparable; but they may serve to complete each other. 



