1871.] 



Successive Polarization of Light. 



385 



analyzer is moved round, the bands advance toward or recede from the 

 thin edge of the wedge without any changes occurring in the colours or in- 

 tensity of the light, the same tint occupying the same place at every half 

 revolution of the analyzer. If the bands advance toward the thin edge of 

 the wedge, the successive polarization of each point is left-handed ; and if 

 they recede from it the succession of colours is right-handed ; every cir- 

 cumstance, therefore, that with respect to a uniform film changes right- 

 handed into left-handed successive polarization, in a wedge of the same 

 substance transforms receding into advancing bands, and vice versa. 

 These phenomena are also beautifully shown by concave or convex films of 

 selenite or rock-crystal, which exhibit concentric rings contracting or 

 expanding in accordance with the conditions previously explained. 



4. Few experiments in physical optics are so beautiful and striking as 

 the elegant pictures formed by cementing laminae of selenite of different 

 thicknesses (varying from -^nnr to inr °^ an mcn ) between two plates of 

 glass. Invisible under ordinary circumstances, they exhibit, when exa- 

 mined in the usual polarizing-apparatus, the most brilliant colours, which 

 are complementary to each other in the two rectangular positions of the 

 analyzer. Regarded in the instrument, fig. 1, the appearances are still 

 more beautiful ; for, instead of a single transition, each colour in the. 

 picture is successively replaced by every other colour. In preparing such 

 pictures it. is necessary to pay attention to the direction of the principal 

 section of each lamina when different pieces of the same thickness are 

 to be combined together to form a surface having the same uniform tint ; 

 otherwise in the intermediate transitions the colours will be irregularly 

 disposed. 



5. A plate of rock-crystal cut perpendicular to the axis loses its suc- 

 cessive polarization, and behaves exactly as an ordinary crystallized film 

 through which rectilinear polarized light is transmitted. 



6. A thick plate of unannealed glass undergoes a series of regular 

 transformations, the principal phases of which are shown, fig. 5. 



V. 



The phenomena of successive or rotatory polarization I have experi- 

 mentally demonstrated admit of a very simple explanation. 



The polarized light incident on the crystallized plate is resolved into two 

 portions of equal intensity, polarized at right angles to each other, one in 

 the principal section, the other perpendicular thereto. These resolved 

 portions, when they fall on the silver plate, have their planes of polariza- 

 tion each at an azimuth of 45°, one to the right, the other to the left of 

 the plane of reflection. These are again resolved in the plane of reflection 

 and the plane perpendicular thereto, and are, in consequence of the unequal 

 retardation, which in silver at an angle of 72° amounts to a quarter of an 

 undulation, converted into circularly polarized beams, one right-handed, 

 the other left>handed. 



