by Potassium Permanganate. 457 



2. Reflection Spectra. — With unpolarized light, and still more 

 with light polarized in the plane of incidence, the dark bands 

 in the spectrum of the reflected light are never very distinct. 

 I was not able to observe whether the bands shifted or not 

 as the angle of incidence increased, as the amount of white 

 light reflected at angles of 55° and upwards was so great as to 

 render the bands invisible. They appeared, however, as long 

 as they were visible, to coincide exactly with the bright spaces 

 in the absorption spectrum of a dilute solution of potassium 

 permanganate, which was thrown into the field by means of 

 the reflecting prism. 



When the incident light is polarized perpendicularly to the 

 plane of incidence, the dark bands are far more distinctly seen. 

 At angles of less than 40° there are four bands, and the blue 

 end of the spectrum is very weak. As the angle of incidence 

 increases, the intensity of the blue rays diminishes; and then the 

 amount of light in the red decreases ; and at about 55° nearly 

 the whole of the light comes from the bright bands. 



As the angle of incidence increases beyond this amount, the 

 dark bands gradually move towards the blue end of the spec- 

 trum; and at about 60° a new band appears near D. With any 

 further increase of the angle more of the blue rays are reflected; 

 and the bands fade away, those in the more refrangible part 

 of the spectrum disappearing first. The relative intensity of 

 the dark bands varies with the angle of incidence. When this 

 is small, the third and fourth bands, counting from the red end, 

 are darkest ; with the increase of the angle the second, the 

 first, and finally the new band, become successively darkest. 



I have not been able to obtain any satisfactory measure- 

 ments of the amount of the displacement of the bands, as, 

 when a spectroscope of sufficient power to render it an easily 

 measurable quantity is used, the bands become so ill-defined 

 that it is impossible to measure them. Approximately the 

 displacement amounts to about *006 in "tenth-metres;" and 

 the bands tend to coincide with the dark bands of the absorp- 

 tion spectrum, instead of with the bright bands as they do 

 when the angle of incidence is about 55° or less. 



The shaded portions of the diagrams are intended to give 

 the relative amount of light, as determined by eye estimations, 

 in the different portions of the absorption and reflection 

 spectra of potassium permanganate — the ordinates being taken 

 to represent the intensity, and the abscissa wave-lengths. The 

 curved line gives the intensity of the light in the different 

 portions of the normal spectrum, as determined by Mossotti * 

 from Fraunhofer's measurements, neglecting the minor irre- 

 gularities in the curve as given by him. 

 * Pogg. Ann. lxxii. p. 509. 



