92 INFRA-RED REFLECTION SPECTRA. 



The refractive index of stibnite, found by Drude, for sodium light is 



449 (5- 1 ?)- 



The value calculated from the well-known Fresnel formula : 



for the region from 4 to 8 /x, where the reflecting power is R = 36, 

 w = 4.o, and n 2 =i6. The dielectric constant, determined for me by 

 Dr. N. E. Dorsey by the static method, by placing thin sections in a 

 small air condenser, is about 8 to 9, which is only about one-half the 

 computed value, -= n 2 = 16. 



The difference is so great that it is not permissible to consider this 

 substance a transparent non-conductor. Of course, if there is a large 



reflection band beyond 15/11 (fig. 55) the above assumption is not true. 

 The proper test would be to find the reflecting power for Hertz waves. 



Konigsberger and Reichenheim 1 examined a series of natural sul- 

 phides by means of what may be called spectrum energy screens or 

 filters, viz, plates of quartz, fluorite, etc., which transmit only certain 

 regions of the spectrum. From the thickness of the plates of the min- 

 erals examined and the observed energy transmitted they computed 

 the reflecting power. The method can not be as accurate as the present 

 one, which involves a direct measurement of the reflecting power ; and 

 this may account for their higher values. For stibnite they observed a 

 reflecting power of 47.6 per cent for the region 0.5 to 4.0 /A, and 43.9 

 per cent for the region 1.6 to 4.0 /z. Of course, it is possible that their 

 sample had a higher reflecting power. The electrical conductivity was 

 of the order icr 15 (Hg= i), and they concluded, as was found in the 

 present work, that the Maxwell relation does not hold true. 



(From Rio Marina, Island of Elba. Curve b, fig. 80.) 



The reflecting surface of one specimen was formed by grinding a 

 large crystal face, which was found would not take a high polish. 

 This specimen gradually increased in reflecting power from 25 per cent 

 at 2 /u, to 34 per cent at 12 //.. The low reflection at 2 p is evidently due 

 to lack of polish. 



Curve b is for a natural cubical crystal face which was i by 1.3 cm. 

 in area. It was quite plane, and had a high polish, except at one end, 

 where there were a few striations. A perfect crystal would have a 



1 Konigsberger and Reichenheim : Centralblatt fur Mineralogie, Jahrg. 1905, p. 

 465- 



