by Potassium Permanganate, 455 



Professor Stokes found (Phil. Mag. vi. 1853, p. 400) that 

 the reflected light contained four bright bands, corresponding 

 in position to the dark bands of the absorption spectrum of a 

 solution of the substance, and that when the reflected light 

 was separated into two streams polarized in, and perpendicular 

 to, the plane of incidence, and then examined by a prism, the 

 bands were hardly visible in the one, and the other at a certain 

 angle consisted mainly of them. 



E. Wiedemann has recently published (Pogg. Ann. cli. 

 1874, p. 625) an account of some experiments he has made 

 on the same subject. He found that whilst the dark bands of 

 the reflection spectrum did not even partially cover those of 

 the absorption spectrum, they did not lie exactly intermediate 

 between any two of them — and, further, that the position of 

 the bands was independent of the angle of incidence, both with 

 ordinary light, and with that polarized in the plane of inci- 

 dence ; but with light polarized perpendicularly to this plane, 

 the bands occupied the same position up to a certain angle, 

 and then with a slight increase of the angle suffered sudden 

 displacement towards the blue, and a new band appeared near 

 D. He also found that with light polarized perpendicularly 

 to the plane of incidence, the position of the bands was inde- 

 pendent of the nature of the surrounding medium, being the 

 same when the permanganate was in air, benzene, and bisul- 

 phide of carbon ; but when the light was polarized in the plane 

 of incidence, with the increase of the refractive index of the me- 

 dium the bands were more and more displaced towards the blue. 



For some experiments I have made on the same subject 1 

 have used a Babinet's goniometer, which has, in addition to the 

 ordinary horizontal stage, a vertical one so arranged that the 

 reflecting surface can be placed over the axis of the instru- 

 ment. Sunlight was used, which could be polarized in any 

 plane by a Nicol supported by the fixed arm of the goniometer ; 

 and a small direct-vision spectroscope, by Hilger, with a 

 " bright-point " micrometer and a reflecting prism for bring- 

 ing a second spectrum into the field, was carried by the other 

 arm of the goniometer. By placing a beaker on the horizontal 

 stage, and, after the surface of the permanganate had been pro- 

 perly adjusted, filling it with the liquid and limiting the inci- 

 dent beam by a narrow vertical slit, the light reflected from 

 the surface of the substance when immersed in a liquid could 

 be examined. 



The experiments were usually made with potassium per- 

 manganate crushed, and burnished with an agate on a piece of 

 finely-ground glass ; and it was found that the light reflected 

 from the surface of crystals and from that of the substance 

 rubbed on glass was identical ; except that ihe blue rays were 



