SGG FOURTH REPORT— 1834. 



white. For the same reason the whole system will he comple- 

 mentary to that which it was before. Professor Airy was led 

 to anticipate this result from the consideration of Fresnel's ex- 

 pressions, and afterwards verified it on trial *, — apparently 

 without any knowledge of the facts observed by M. Arago. A 

 similar confirmation of the same principles may be obtained by 

 combining, in Fresnel's experiment, a metallic reflector with 

 one of glass. The light being polarized perpendicularly to the 

 plane of reflexion, the central band will be icJdte when the 

 angle of incidence is below the polarizing angle of the glass ; at 

 the polarizing angle the interference bars will vanish altogether ; 

 and beyond that incidence they will reappear with a dark cen- 

 tre, instead of a white one. This method of observation would 

 seem to be peculiarly adapted to the investigation of the change 

 of phase produced by metallic reflexion at various incidences. 



By the same considerations Professor Airy was led to expect 

 that when Newton's rings were formed between two transparent 

 substances of diiferent refractive powers, — the light being polar- 

 ized perpendicularly to the plane of incidence, — the rings should 

 be black-centred, when the incidence was less than the polarizing 

 angle of the low- refracting substance, or greater than that of 

 the high-refracting substance : while they should appear with 

 a ivhite centre, when it was intermediate to these angles ; — the 

 vibrations of the waves reflected from the two surfaces being of 

 opposite signs in the former case, and of the same sign in the 

 latter. All these expectations were fully confirmed by experi- 

 ment t. The substances selected by Professor Airy for these 

 observations were plate-glass and diamond, — these substances 

 diflfering verj" widely in their refractive powers; and in the 

 course of his experiments he has noticed certain peculiarities in 

 the phenomena, from which he has drawn some highly interest- 

 ing conclusions respecting the nature of reflexion from diamond. 

 Had this been subjected to the ordinary laws, the reflexion 

 should cease, and the rings disappear, at the polarizing angle 

 of both substances. This however was not the case. The rings 

 did not vanish at the polarizing angle of the diamond ; but the 

 first black ring contracted, as the incidence was gradually in- 

 creased, and finally usurped the place of the central white spot. 

 A portion of the light is therefore still reflected at the maxi- 

 mum polarizing angle of diamond ; and it is evident from the 

 phenomenon that the transition from a white to a black centre 

 is owing to n gradual change of jj base oi the reflected vibration, 



• " On a Reniaikable Modification of Newton's Rings," Cambridge Trans. 

 1832. 



+ " On the Plienomena of Newton's Rings, when formed between two trans- 

 parent substances of difl'erent refractive powers," Cambridge Trans. 1832. 



