Modification of Netvton's Rings, 281 



a plane perpendicular to the plane of incidence, the phenomena 

 undergo a remarkable change. When the angle of incidence is 

 small, the central spot is dark, but not black. It continues 

 dark and without any sensible alteration of size as the angle 

 of incidence approaches to the polarizing angle of the glass. 

 The rings become faint (the central dark spot retaining the same 

 magnitude as long as it is visible) and disappear* when the 

 angle of incidence is equal to the polarizing angle. On increas- 

 ing the angle of incidence by a very small quantity the rings 

 are again seen, of the same magnitude, but the central spot 

 without any sensible alteration of size is now white. And the 

 intensity and colour of the rings appear to be, in every part, 

 complementary to what they were before. This state continues 

 with very little alteration in the magnitude of the central white 

 spot, till the angle of incidence becomes 90°. 



If the light is polarized before or after incidence in the plane 

 of reflexion, there is no such change of colours. The magnitude 

 of the central spot is altered ; but through all variations of the 

 angle of incidence the central spot still remains darker than the 

 ring which immediately incloses it. 



If common light is incident at an angle greater than the 

 polarizing angle, and a plate of tourmaline is held between the 

 glass and the eye, the axis of the tourmaline being in the plane 

 of reflection, the central spot is black. On turning it to the right 

 or the left, the dark spot dilates and a white spot arises in the 

 center, which acquires its maximum diameter when the axis of 

 the tourmaline is perpendicular to the plane of reflexion. On 



* This simple fact (the disappearance of the rings while abundance of light is re- 

 flected from the metal) seems to be satisfactory evidence, if any were wanted, to shew 

 that the rings are produced by interference only. 



Vol. IV. Part II. N n 



