382 



Sir C. Wheatstone on the 



[Mar. 23, 



bifurcation which a ray of polarized light suffers when it is transmitted 

 through a crystallized plate, is a very appropriate one; but as there are 

 different kinds of such separation, we may designate plane dipolarization 

 the resolution into two plane-polarized rays at right angles to each other, 

 and circular dipolarization the resolution into two circularly polarized 

 rays, one right-handed, the other left-handed. In like manner the term 

 elliptic dipolarization may be employed to represent the phenomena 

 shown by transmitting a polarized ray through a plate of rock-crystal 

 obliquely to the axis. 



The object of the present communication is to make known another 

 means of producing successive polarization, both right-handed and left- 

 handed, which, equally with the well-known modes, may be proved to 

 arise from the interference of two opposite systems of circularly polarized 

 rays. 



III. 



The polarizing-apparatus which I have employed for the experiments 

 I am about to detail is represented by PI. IV. 



A plate of black glass, G, is fixed at an angle of 3° to the horizon. The 

 film to be examined is to be placed on a diaphragm, D, so that the light 

 reflected at the polarizing-angle from the glass plate shall pass through it 

 at right angles, and, after reflection at an angle of 18° from the surface of a 

 polished silver plate S, shall proceed vertically upwards. N is a Nicol's 

 prism, or any other analyzer, placed in the path of the second reflection. 

 The diaphragm is furnished with a ring, moveable in its own plane, by 

 which the crystallized plate to be examined may be placed in any azimuth. 

 C is a small moveable stand, by means of which the film to be examined 

 may be placed in any azimuth and at any inclination ; for the usual expe- 

 riments this is removed. 



If a lamina of quartz cut parallel to the axis, and sufficiently thin to 

 show the colours of polarized light, be placed upon the diaphragm so that 

 its principal section (i. e. the section containing the axis) shall be 45° to 

 the left of the plane of reflection, on turning the analyzer from left to 

 right, instead of the alternation of two complementary colours at each 

 quadrant, which appear in the ordinary polarizing apparatus, the phe- 

 nomena of successive polarization, exactly similar to those exhibited in the 

 ordinary apparatus by a plate of quartz cut perpendicular to the axis, 

 will be exhibited ; the colours follow in the order R, O, Y, G, B, P, V, 

 or, in other words, ascend as in the case of a right-handed plate of 

 quartz cut perpendicularly to the axis. If the lamina be now either in- 

 verted, or turned in its own plane 90°, so that the principal section shall 

 be 45° to the right of the plane of reflection, the succession of the colours 

 will be reversed, while the analyzer moves in the same direction as before, 

 presenting the same phenomena as a left-handed plate of quartz cut per- 

 pendicularly to the axis. Quartz is a positive doubly refracting crystal ; 

 and in it consequently the ordinary index of refraction is smaller than the 



