﻿400 Royal Institution. 



begin to appear, and will be brightest when the angle of turning 

 amounts to 90°. The colour depends upon the thickness of the 

 crystal ; so that by a suitable preparation any arrangement of colours 

 may be produced. 



So far for plane polarization. The principal of circular or succes- 

 sive polarization, as regards the present purpose, is as follows : — 



If two sets of rectilinear vibrations lying in planes perpendicular 

 to one another meet and combine, the resulting vibration will be 

 curvilinear, whose form and position depends upon the difference of 

 phase of the components. If the second set be in advance or in rear 

 of the first by a quarter of a wave-length, the resulting vibration 

 will be circular ; but the motion will in one case be direct (like the 

 hands of a watch), in the other it will be reverse. 



If two sets of circular vibrations in opposite directions meet and 

 combine, the resulting vibrations will be rectilinear, and the position 

 of their plane will depend upon the difference of phase of the compo- 

 nents. If the second set advance upon the first, the plane of resultant 

 vibration will undergo direct rotation ; if it recede, it will undergo 

 reverse rotation. 



If in such an experiment white light be used, the vibrations of the 

 different component prismatic colours will (on account of their dif- 

 ferent wave-lengths) undergo different retardation ; and consequently 

 the resultant vibrations will lie in different planes, arranged in pris- 

 matic order. The order will be from red to violet, or vice versd> in 

 accordance with the law stated above. 



If a ray of plane-polarized light fall upon a metallic reflector, it 

 is divided into two, whose vibrations are respectively parallel and 

 perpendicular to the reflector ; and the latter is retarded behind the 

 former by a difference of phase depending upon the angle of inci- 

 dence. If the plane of vibration of the incident ray be inclined at 

 an angle of 45° to the plane of incidence, the two rays into which it 

 is divided have nearly the same intensity. At an angle, nearly 45°, 

 which varies with the metal employed, but which is perfectly definite, 

 the intensities become accurately equal. And, further, if the angle 

 of incidence have a particular value, dependent upon the nature of 

 the metal (for silver 72°), the retardation will amount to a quarter 

 of a wave-length. These two rays, on leaving the reflector, will re- 

 combine, and, in accordance with the laws above given, will in the 

 last-mentioned circumstances become a circularly polarized ray. 

 Lastly, the direction of motion in this circular ray will depend upon 

 the side on which the original plane of vibration is inclined to the 

 plane of incidence ; if, when it is inclined on one side, the circular 

 ray becomes right-handed, then, when it is inclined on the other, 

 it becomes left-handed. 



Reverting then to the phenomena of double refraction, produced 

 by a plate of crystal cut parallel to the axis on a plane- polarized ray, 

 let the crystal be placed in such a position that the planes of vibra- 

 tion of the two resulting rays are inclined at angles of 45° on the 

 two sides respectively of the plane of incidence ; and let there be 

 interposed between the crystal and the analyzer a silver plate at an 



