26 Prof. Airy on the Phenomena of Newton's Rings, as formed 



of incidence, the first set of rings again disappeared, and re- 

 appeared in great brilliancy, the centre being now black. 



I am willing to think these experiments important, because 

 they bear immediately upon a part of Fresnel's theory which 

 has always appeared to me most liable to objection, namely, 

 the formulae for the extent of vibration in reflected and re- 

 fracted rays. On the truth of Fresnel's general theory as a 

 mere geometrical representation, namely, that light consists of 

 transversal vibrations, and that polarized light is light in which 

 all the vibrations are perpendicular to the plane of polarization, 

 I shall say nothing, because I do not think it will be doubted 

 by any one who is well acquainted with the experiments and has 

 examined their agreement with calculation. But on the theo- 

 rems for intensity in reflected rays, &c. involving points of the 

 greatest obscurity, and supported only by very forced suppo- 

 sitions, any one may I think with reason be sceptical. The 

 phenomena described here and those described in a former 

 paper (On a remarkable Modification, &c.) depend entirely, 

 in theory, upon the changes of sign of certain quantities which 

 enter into Fresnel's expressions for these intensities. With 

 respect to the absolute measure of the intensities I can say 

 nothing, except that the general appearance of the brightness 

 is sufficiently in accordance with the lav/. On the whole I 

 think that these experiments give great probability to the 

 truth of the formula considered as a general law : and that 

 they establish with certainty that part of it which implies that, 

 after passing a certain angle, the direction of the vibration in 

 the reflected ray (considered with respect to that in the inci- 

 dent ray) is reversed. 



Observatory, Feb. 4, 1832. G. B. AlRY. 



Postscript. — Since the above account was written I have 

 (with a favourable sky) seen the white-centred rings many 

 times, and several times with a doubly-refracting prism only, 

 unassisted by a tourmaline. In examining one part of the 

 phenomena, I find that there is a discordance of a most curious 

 kind from what the strict theory had led me to expect. 



When the light is incident at the polarizing angle of the 

 glass, the rings, so far as I can see, vanish totally. Though I 

 have looked several times with the most scrutinizing attention, 

 I have not been able to see the least trace. If the angle of 

 incidence is gradually increased till it exceeds the polarizing 

 angle, the black-centred rings disappear gradually without 

 altering their size (a considerable quantity of light being still 

 reflected from the diamond) and white-centred rings of the 

 same size appear in their place, without any intermediate 



