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 XXXIII. Intelligence and Miscellaneous Articles. 



ON THE LIGHT REFLECTED BY PERMANGANATE OF POTASSIUM. 

 BY BR. EILHARD WIEDEMANN. 



PBOEESSOK STOKES* observed that in the spectrum of the 

 light reflected from solid permanganate of potassium dark 

 streaks occur, and that they are exhibited most distinctly with a 

 certain angle of incidence ; further, the minima of brightness in the 

 spectrum of the reflected light correspond to the rays transmitted 

 in the greatest intensity by the permanganate. 



I hare pursued this subject further, and examined not only the 

 light reflected at the boundary of permanganate of potassium and 

 air, but also that at the boundary of benzine, sulphide of carbon, 

 and a mixture of these two substances, and the above salt. More- 

 over the polarization of the incident light was kept in view. To 

 obtain the reflecting surfaces, triturated crystals of the salt were 

 polished upon ground glass plates by means of a jet-burnisher. 

 Clean surfaces, free from oxide, were thereby secured for the inves- 

 tigation, which is not the case when whole crystals are employed. 

 The glass plate thus prepared was inserted in a rectangular hollow 

 prism (which could be filled with the different liquids) in such wise 

 that its coated face was turned to the rectangular edge. The prism 

 was placed upon a graduated circular table that could be rotated, 

 and sunlight so thrown upon one of the two surfaces including the 

 right angle that the light refracted there fell upon the coated plate 

 and, through reflection, passed out at the other surface. Thence it 

 arrived at the slit of a spectrum-apparatus. The angle of incidence 

 on the coated plate was determined thus : the light from the first 

 surface of the prism was reflected back in its own direction ; the 

 position of the table was then read off ; the rotation of the table 

 with the prism gives immediately the incidence-angle at the first 

 surface ; from this angle and that between the glass plate and the 

 first face of the prism, and the index of refraction of the medium 

 in contact with the permanganate, the incidence-angle at the latter 

 can then be found. 



The position of the streaks in the spectrum was determined by 

 means of a photographed scale applied to the spectrum-apparatus, 

 the cross-threads of the observing-telescope haviug previously been 

 placed on the centre of the dark streak. 



These positions with pretty large angles of incidence are given 

 in Table I. The columns under A refer to the streaks in the light 

 polarized parallel to the plane of incidence, those under B to those 

 in the light polarized perpendicular to that plane. The first column 

 gives the names of the surrounding media. Table II. gives the 

 positions of the absorption-streaks in the transmitted light. Fraim- 

 hofer's lines correspond as follows to the strokes on the photo- 

 graphed scale : — 



D = 0; E = 18; 5=21; E = 33. 



* Phil. Mag. 1853, vol. vi. p. 393. Pogg. Ann. 1854, vol. xci. p. 300. 



