288 Professor Airy on a remarkable Modification, Sfc. 



is small. I have submitted this to experiment, but I have not 

 been able to discover with certainty any alteration in the size of 

 the rings. 



The reasoning- upon which the principal experiment described 

 in this paper was anticipated, may probably be applied in many 

 similar cases. The following is a very remarkable instance. If 

 a lens of a low-refracting substance be placed on a plate of a 

 highly-refracting substance, or vice versa, and if they be illumi- 

 nated with light polarized perpendicular to the plane of incidence, 

 I expect that while the angle of incidence is less than the pola- 

 rizing angle of the low-refracting substance, the central spot will 

 be dark ; when the angle of incidence is greater than the polarizing 

 angle of this substance and less than that of the other, the central 

 spot will be bright; and when the angle of incidence is greater 

 than the polarizing angle of the highly-refracting substance, the 

 central spot will again be dark. I have not yet procured any 

 substances proper for the verification of this conclusion. 



G. B. AIRY. 



Observatory, 

 June 21, 1831. 



