JEAN BECQUEREL and H. KAMEELINGH ONNES. On the absorptionspectra 

 of the compounds of the rare earths at the temperatures obtainable 

 by liquid hydrogen and their change by the magnetic field. 



PI. IV. 

 642.3 654.3 



7^=20°abs. 



r-=i4° „ 



Fig. 5. Magnetic rotation of the plane of polarisation. Xenotime 2^ spectrum 

 (Rowland grating); thickness 0.80 mM., field 18000 Gauss; (quarter of wavelength 

 plate turned 90° in the one in respect to the other). 



r= 14° abs. 



r= 14" 



r=14° 



T= 14" 



Fig. 6. Sulphate of Neodyme group in the orange, 2d spectrum ((Rowland grating) 

 spectra of the vibrations : 



I in the principal direction a, field = 0. 



b, field =0. 



a, a and b normal to the field (18000 Gauss). 



b, a and b „ „ „ „ „ 



a, a and c „ „ „ „ „ 



c, a and c „ „ „ „ „ 



b, b and c „ „ „ „ „ 



c, b and c „ „ „ „ „ 



