145 
Normal Pyrrhotine (Fe,S, = (FeS), Fe,S,, from Morro Velho, Brazil). 
The piece with which I made my measurements was polished, in 
the first case, parallel to the magnetic plane and in the second case 
in a plane normal to this and to the direction of easiest magnetisation °). 
-The first mirror, as was to be expected, showed no rotation whatsoever. 
On the second surface however a positive rotation of the order 
of one minute was to be observed for the whole region of the 
spectrum. Even at the ends of the spectrum no indication of an 
inflection in the dispersion curve could be found: (perhaps a trace 
of an increase in the violet). I repeated the same measurements 
thereupon more accurately, the slit of the monochromator being 
widened, using a brighter yellow and blue illumination. The field here 
amounted to at least 12> kgs. 
ie Bole E „10; 
e=f (2) Normal Pyrrhotine (saturated) WEISS 
N > (ww) £L (mm) (min.) + de 
35 435 + 66 | +0,98' | 0,04'= 47, 
30 > rbe Keel 1.00 0,05 = 5 » 
25 466 Oe ead 0,04 = 4 » 
20 483 +66 | +097 | 0,04=4> 
20 503 + 64 | +095 | 0,03=3> 
20 530 E650. ln 0196 0.03 =3> 
20 567 +°6,4_ |=! 0,95 0,02 = 2> 
20 615 6.68 e090 0,03 = 3 » 
25 675 +63 | +0,94 0,04 = 4 » 
30 3 166 | 1007 00335 
20 blau + 6,4 + 0,94 0,02 = 2» 
9001 > 4-66 | 4096 | 001 == 1» 
20 | gelb | +64 | 40,95 0,02=2. 
20 > +65 '.+096 | DO 15 
Compact magnetic pyrite (presumably from Obermais, Tyrol). A 
naturally reflecting surface was previously found by pv Bors to show 
1) According to the above formula this substance ought to be regarded as Sulpho- 
pentaferroferrite. P. Weiss, Journ. de Phys. (4) 4 p. 469, 1905 finds for the 
saturation value of magnetisation about 60 to 75 CGS, 
