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LIV. On the Interference-Rings, described by Haidinger, 

 observable by means of Plates whose Surfaces are absolutely 

 Parallel. By Lord Rayleigh, O.J/., Pres.R.S. * 



[Plate XVI.] 



THE importance which these rings have acquired in 

 recent years, owing to the researches of Michelson and 

 of Fabry and Perot, lends interest to the circumstances of 

 their discovery. It seems to be usually supposed that Hai- 

 dinger merely observed the rings, without a full appreciation 

 of the mode of formation. Thus Mascartf writes : " C'est 

 par ce precede, que Haidinger les a observees le premier 

 avec une lame de mica, mais sans en donner la veritable ex- 

 plication." A reference to the original papers will, I think, 

 show that Haidinger, in spite of one or two slips, understood 

 the character of the rings very well, and especially the dis- 

 tinction between them and the rings usually named after 

 Xewton and dependent upon a variable thickness in the thin 

 plate. 



In the first memoir (Pogg. Ann. lxxvii. p. 219, 1849) the 

 bands formed hx reflexion are especially discussed. A spirit- 

 flame with salted wick, seen by reflexion at considerable 

 obliquity in a mica plate, is traversed by approximately 

 straight bands running perpendicularly to the plane of in- 

 cidence. Talbot had observed phenomena in many respects 

 similar. ... But the yellow and black lines, observed by 

 Talbot in thin blown glass, differ in character from the lines 

 from mica, though both are dependent upon the interference of 

 light. In the case of the glass the interference is due to the 

 fact that the thickness of the glass is variable, and the lines 

 are localised at the plate. The lines from the mica behave 

 differently. However the plate may be turned round in its 

 own plane, the yellow and dark lines remain perpendicular 

 to the plane of incidence. The two surfaces of the mica are 

 absolutely parallel to one another, and accordingly the 

 phenomenon is the same in all azimuths. The lines appear 

 sharper and more distinct, the nearer the mica be held to the 

 eye, in contradistinction to the lines from glass which then 

 become more and more indistinct and finally disappear. 



The bands are due to interference of light reflected at the 

 two surfaces. The difference of path for the rays reflected 

 at the front and back surfaces amounts for the bright bands 

 to a whole number of wave-lengths plus a half wave-length, 



* Coimnumcated "by the Author. 



t Ann. de Chim. t.'xxiii. p. 128 (1871). 



