﻿490 Lord Rayleigh on Interference-Rings observable 



for the dark bands to a whole number of wave-lengths 

 simply. 



The dark parallel lines are seen with the greatest distinct- 

 ness in the reflected light. There is then a striking contrast 

 between the reflected bright light and the black due to its 

 absence, the plate being backed by a dark ground.. If the 

 plate is held in an oblique position between the eye and the 

 flame, the parallel lines are seen directly, but there is a much 

 less striking contrast with the bright parts. 



At this time Haidinger had not succeeded in seeing the 

 complete rings, but in a later memoir (Pogg. Ann. xcvi. 

 p. 453, 1854) he returns to the subject and shows how the 

 obstacle to the incident light caused by the head of the 

 observer may be overcome with the aid of a glass plate 

 inclined at 45°. The incident light on its way to the mica 

 is reflected at the glass plate, while on its return it traverses 

 the plate and so reaches the eye. 



The observation of the transmitted rings is of the simplest 

 possible character. It is sufficient to look through the plate 

 of mica at a sheet of white paper illuminated either from in 

 front or from behind by the homogeneous light of the spirit 

 flame. The rings are complementary to those seen by re- 

 flexion. They are, however, much less intense, being due to 

 the interference of the powerful directly transmitted light 

 with the much feebler light twice reflected in the interior of 

 the plate. 



The distinction between u Beruhrungs-ringe " and " Plat- 

 tenringe" is again emphasised, the former depending upon 

 a variable thickness, the latter upon a variable obliquity. 

 We may well agree with Haidinger when he concludes: 

 " Die Plattenringe am Glimmer bilden also eine Classe von 

 Interferenz-Erscheinungen fur sich, die einfachste, die es 

 geben kann, wie ich diess in der vorhergehendeu Zeilen mit 

 hinreichender Evidenz nachgewiesen zu haben glaube""*. 



It is interesting to remark that Haidinger's rings, rather 

 than Newton's, are those directly explained by the usual 

 calculation due to Young, Poisson, and Airy, where plane 

 waves of light are supposed to be incident upon a parallel 

 plate. The application to a plate of variable thickness cannot 

 be more than approximately correct. That the indirect rather 

 than the direct application should have been (until lately) 



* It should perhaps be rioted that Haidinger omits the factor p. 

 (refractive index) in the expression for the retardation on which the 

 interference depends, viz. '"Ipe cos 9, where e denotes the thickness of the 

 plate and 9 the angle of refraction. Also that, probably by a slip of 

 the pen, he speaks of the retardation as increasing -with the obliquity. 



