280 Professor Airy on a remarkable 



Now instead of common light, suppose that polarized light is 

 incident. The general appearances are not altered ; the only 

 modification being that, if the plane of polarization is perpen- 

 dicular to the plane of reflection, when the angle of incidence 

 becomes equal to the polarizing angle the whole of the reflected 

 light disappears, and with it the whole system of rings. But on 

 increasing the angle of incidence the reflected light again appears, 

 and the system of rings is restored, with the center black as 

 before. It is indifferent whether the light is polarized before 

 incidence, or a polarizing substance (as a plate of tourmaline) is 

 placed between the lenses and the eye. 



Instead of placing the lens on a plate or lens of glass, let 

 it be placed on a polished plate of metal. If common light is 

 used, the rings are seen as before (though not so black), but the 

 central spot is perhaps not quite so large as before. On increasing 

 the angle of incidence up to 90°, the rings dilate as before, but 

 the central spot, I think, diminishes a little (at least in proportion 

 to that formed when the lens is placed on a glass plate). In 

 this case then, the general character of the appearances is almost 

 exactly the same as before. 



If however the lens is placed on a polished plate of metal, 

 and if the light is polarized either before or after incidence* in 



* I have carefully verified this assertion (that it is indifferent whether the light is polarized 

 before or after reflection) because I think that it leads to important theoretical conclusions. 

 If polarization were a modification of light (as Dr. Brewster and others have supposed), it 

 might be conceived that polarization before incidence might destroy its power of producing 

 rings at a certain angle, or might change the tints; but when the reflexion is performed, 

 and the rings are actually visible to the eye with a dark center, it seems quite inconceiv- 

 able that any modification or physical change in the light should make that center appear 

 white. The satisfactory explanation is, that polarization is a resolution of the vibrations into 

 two sets at right angles to each other, performed in such a manner that the two sets can 

 in general be separately exhibited, and that in this instance only one is transmitted to the eye. 



