( 845 ) 



Band I, V and VI vani.slied in llic field: with the field off 

 II showed a dark core with shades on either side, which dis- 

 appeared ill the field. The two doublets VII and VIII gave the 

 iii-eatest resolution measured as yet — 1,5 X (^i — ^i) ; ^'^ith the 

 field used the components facing each other happened to coincide, 

 so that the pair of doublets looked like a very wide triplet with a 

 heavy middle band. It ought to be possible to observe a pfienomenon 

 of this order of magnitude with every good spectroscope. 



Finally we mention that the neodymium-magnesium nitrate of § 9 

 also occurs in hexagonal crystals ; such optically uniaxial crystals 

 are of great interest (§19); there also exists an isomorphous series 

 of salts, which contain manganese, cobalt, nickel or zinc. Measu- 

 rements on this subject have been made, partly they are still in 

 preparation. 



Samarium sulphate [Sm.^{SO^)^ .SH^O]. 



§ 25. We now examined a more transparent sample (cf. § 19), 

 which again contained both optical axes, and was placed like the 

 other sulphates; 4 bands in the green were measured at — 193°. 



40 Kilogauss 



Plane of Polarisation horizontal 



Thickness 0.8 mm. 



The effect was apparently small here ; the widened bands were 

 vague. With a vertical plane of polarisation the phenomenon scarcely 

 changed. 



Erhimni/Wium mlphate \{Er,Y)^{SO^)i\ 



§ 26. We also examined an impure product obtained by treatment 

 of the original minerals with sulphuric acid, in which erbium and 

 yttrium occur in variable percentages, and the latter preponderates : 

 the crystals were monoclinic. The group of bands in the green, 

 yellow-green, and red showed peculiar and intricate eflecis of mao- 



57 - 

 Proceedings Royal Acad. Amsterdam. Vol. X. 



