142 Prof. Airy's New Optical Experiments. 



plane glass it is well known that a set of rings is seen whose 

 centre is remarkably black : and it is indifferent whether com- 

 mon light or polarized light be used, the only difference made 

 by the latter being that when the plane of polarization is per- 

 pendicular to the plane of reflection, and the angle of inci- 

 dence is the polarizing angle, the rings disappear; but on 

 altering the angle of incidence either way, the rings still ap- 

 pear with the centre black. If, however, a lens is placed on 

 a metallic surface, and the incident light is polarized in the 

 plane perpendicular to the plane of incidence; while the angle 

 of incidence is small, the centre of the rings is black ; when it 

 is equal to the polarizing angle of the glass the rings disappear 

 (though there is still copious reflection from the metal) : tlieo 

 on the smallest increase of the angle of incidence the rings 

 are seen with their centre 'white, and they continue so till the 

 angle of incidence = 90°. It is indifferent whether the light 

 is polarized before or after reflection ; and a remarkable effect 

 may be thus produced : if common light is incident at an angle 

 greater than the polarizing angle, the rings have a dark 

 centre ; but on placing a plate of tourmaline (with its axis 

 perpendicular to the plane of reflection) between the eye and 

 the lens, the rings are seen with a bright centre. The Pro- 

 fessor conceives that the whole of these experiments are in 

 the highest degree favourable to the theory of undulations 

 with transversal vibrations as given by Fresnel, and to the idea 

 (which is a necessary part of that theory) that polarization is 

 not a modification or physical change in the light, but a reso- 

 lution of its vibrations into two sets, one in one plane, and the 

 other in the plane perpendicular to the former, one of which 

 sets sometimes is suppressed and sometimes describes a dif- 

 ferent path. The last experiment (where the character of the 

 rings is changed after they are formed) appears almost de- 

 cisive of this point. From the manner in which the rings 

 alter when the tourmaline is turned, the Professor infers that 

 the phases of the vibrations in the plane of reflection are more 

 accelerated by reflection at metallic surfaces than those of the 

 vibrations perpendicular to the plane of reflection. The dark 

 centres, it is to be observed, are never so dark as when the 

 lens is placed on glass ; and the bright ones are never very 

 bright. 



The result of the following experiment, like those of the 

 former,was anticipated by theoretical considerations, and shows 

 the clearness with which, by Fresnel's theory, the effects of 

 modifications can be traced whose very nature is inexpressible 

 on any other theory. In the common polarizing apparatus, 



plane- 



