322 



reflected the blue light at all, so that it was impossible to use wave- 

 lengths shorter than 483 mt. 



450 



500 



550 600 



Fia. 2. 



650 



700 



On heating, the surface near 80° lost its reflecting power. After 

 repolishing this occurred again, so that no temperature curve could 

 be determined. 



Tro?i carbide III (157„ 0, Hilpert, & = 183°). Here the rotation 

 was smaller (between — 4' and — 14') and showed a numerical 

 minimum in the blue-green. Yellowishness of the mirror, however, 

 prevented measurements in the blue below 466 /ifi. 



The dispersion curve is shown in Fig. 3. 



• 3' 



- 4' 



- 5' 



- 6' 



- 7' 



- 8' 

 ■ 9'- 



10'- 

 -11' 

 -12' 

 -13' 

 -14' 

 -15' 



Fe, C 

 (15 0/0 C.) Ill 



.= f(/) 



J L 



_l__L 



I I I I I I 



450 



500 550 



Fi2. 3. 



600 



650 



Above 110° the rotation decreased graduall}' and at a nearly 

 linear rate up to about 183°. The reflected image finally became 

 indistinct and during the cooling only a small rotation of 1,5' could 

 with difficulty be perceived. This observation was repeated. Gentle 

 polishing, however, served to restore the original condition completely, 

 so that the phenomenon cannot be regarded as thermal hysteresis. 



The temperalui-e curve is shown in Fig. 4. 



The zero point agrees with that found by Woi-ogdin. According 



