of Newton's Rings. 40 \ 



be considered as the effect of a retardation in the phase of the 

 light reflected from the lower surface. The contraction of the 

 rings then in passing- the polarizing- angle of the diamond requires 

 us to admit that the phase of the reflected light (the incident 

 light being polarized perpendicular to the plane of the reflexion) 

 is on increasing the angle of incidence by a few degrees, retarded 

 nearly 180'. 



The retardation however is not quite 180°. For if it were, the 

 character of the rings would be exactly changed, so that the pro- 

 portion of the size of the central black spot to that of the first 

 white ring would be the same as that of the central white spot 

 (before the change) to the first black ring. But as the central 

 black spot formed by rays polarized perpendicular to the plane 

 of reflexion is distinctly larger than that formed by rays polarized 

 in the plane of reflexion, it seems that the black ring has not 

 contracted completely, or that the alteration of phase is not quite 

 180°. This reasoning it must be confessed is not certain, as the 

 same thing would be explained by supposing a small alteration 

 of phase in the light polarized in the plane of reflexion. I may 

 mention here that in the Newton's rings formed between two 

 lenses of the same kind of glass, the central black spot in those 

 formed by light polarized perpendicular to the plane of reflexion 

 is larger than in those formed by the light polarized in the plane 

 of reflexion. 



If, while the white-centered rings are under examination, the 

 tourmaline and doubly-refracting prism are turned round, the 

 rings become faint, but do not disappear, and are changed int., 

 black-centered rings by the contraction of the rings. This is 

 exactly similar to what takes place when a lens is placed on a 

 metallic surface, and it proves that (as in the former paper). 



y 11 2 



