﻿492 Lord Rayleigh on Interference-Rings observable 



complete discordance would not occur until a thickness of 

 *80 mm. was reached. Practically in both cases the ring- 

 systems may be considered to be in coincidence. But 

 although the duplicity of the soda line is not the cause, 

 there is in fact a second ring-system, owing to the double 

 refraction of the mica. That this is the case is easily proved 

 with the aid of a nicol capable of rotation about its axis. 

 When the ring-system of the thicker plate is examined with 

 the nicol, there are four portions at right angles to one another 

 at w T hich the inner rings become distinct. But in adjacent 

 positions, i. e. positions distant 90°, of the nicol the ring- 

 systems seen are different. If one has a bright centre, the 

 other has a dark centre. When no nicol is used, or when 

 the nicol occupies positions at 45° to those above mentioned, 

 the ring- systems interfere, and little or nothing is visible, at 

 any rate near the centre. When the thinner plate is employed, 

 the ring-system is really double, but does not appear to be so, 

 since the components are approximately in coincidence. In 

 this case the appearance is but little altered by the use of a 

 nicol however held. 



It is only near the centre of the system that the rings are 

 obscured when the thicker plate is used without a nicol. 

 Further out, the rings become distinct enough. A closer 

 examination shows, however, that this statement needs quali- 

 fication. Along four directions, apparently at right angles, 

 radiating from the centre, there are regions of no definition. 

 These regions are narrow, so narrow that they might at first 

 escape observation, and they constitute, as it were, spokes of 

 the ring-system. It was natural to suppose that these spokes 

 represented places where the rings of each system bisected 

 the intervals between the rings of the other system — a con- 

 jecture supported by the fact that the spokes disappeared 

 when a nicol was introduced in the positions suitable for ren- 

 dering the inner rings distinct. The effect was to make distinct 

 the outer rings also all round |the circumference. Further 

 confirmation was afforded by the introduction behind the 

 mica of cross- wires and a collimator-lens, serving to indicate 

 a fixed direction. This was pointed at a spoke, so that without 

 a nicol no bands were visible in the neighbourhood of the 

 cross-wires. The nicol was then introduced in such a position 

 as to give maximum distinctness, and the cross-wires adjusted 

 to coincide with the centre of a bright band. A rotation of 

 the nicol through 90° showed that in the band-system then 

 visible the cross- wires marked the centre of a dark in place of 

 a bright band. 



