9& REPORT OX PHYSICAL OPTICS. 



of the central white spot. A portion of the light is therefore still 

 reflected at the maximum polarizing angle of diamond ; and it is 

 evident from the phenomenon that the transition from a white to 

 a black centre is owing to a gradual change of phase of the reflected 

 vibration, amounting to nearly 180, while the coefficient of the 

 vibration itself is not much altered. The diamond therefore has 

 no angle of complete polarization ; and Professor Airy concludes 

 that the nature of the reflexion from this singular substance, in 

 the neighbourhood of the angle of maximum polarization, is differ- 

 ent from any that has been hitherto described. 



Fresnel's theory of reflexion has received experimental confir- 

 mation of a different kind, and to an extent which leaves little 

 ground to doubt of its truth. When a ray polarized in any plane 

 falls upon a reflecting surface at any angle, the reflected ray is 

 still polarized, but its plane of polarization is changed the 

 amount of the change depending on the incidence. The law of 

 this change is at once furnished by the theory of Fresnel ; for the 

 tangent of the inclination of the plane of polarization of the re- 

 flected ray to the plane of incidence is equal to the ratio |of the 

 displacements in the plane of incidence and in the perpendicular 

 plane. The formula thus deduced has been verified in the most 

 complete manner by the observations of Fresnel himself, and more 

 fully since by those of M. Arago and Sir David Brewster.* 



The views of the latter philosopher respecting the nature of 

 partially-polarized light are founded upon the phenomenon of the 

 change of the plane of polarization by reflexion. If common light 

 be conceived to consist of two pencils oppositely polarized, in planes 

 inclined 45 on either side of the plane of reflexion, the effect of 

 reflexion, it is obvious, will be to bring each of these planes nearer 

 to the plane of incidence ; so that the planes of polarization of the 

 two pencils will approach each other, and form an acute angle 

 after reflexion. Partially-polarized light, then, according to Sir D. 

 Brewster, consists of two polarized pencils, whose planes of polari- 

 zation form an acute angle; and no portion of it is in the condition 

 of ordinary light.f This hypothesis receives some support from 



* Annales de Chimie, torn. xvii. ; Phil. Tram. 1830. 



t Sir David Brewster has computed, on these principles, the quantity of light ap- 

 parently polarized in the plane of incidence, hy a single reflexion at-any angle, adopt- 

 ing Fresnel's expression for the intensity of the reflected ray. The agreement of the 



