﻿on Plates ivhose Surfaces are absolutely Parallel. 493 



The disappearance o£ the rings at the places where the 

 brightest places in one system bisect the intervals between 

 the brightest places of the other system depends of course 

 upon the width of the bright rings being not much less than 

 half of the complete period. If, as in the Fabry-Perot appa- 

 ratus, the bright rings are much narrower, both systems should 

 become visible. I thought therefore that it would be of 

 interest to silver lightly on both sides a portion of the thicker 

 plate, the more as, apart altogether from the spokes, the whole 

 effect would be improved owing to the enhanced reflexions. 

 By the chemical method, as ordinarily used for silvering glass 

 surfaces, I did not succeed ; but there was no difficulty in 

 getting the required deposits by the method of electrical 

 discharge in vacuo using a silver cathode *. With the 

 silvered plate the nature of the whole phenomenon, including 

 the character of the spokes, was evident on simple inspection. 

 At the spokes both ring-systems could now be seen, forming 

 a compound system of half the original period. If we neglect 

 the circular character of the rings, the effect may be imitated 

 with straight {bands, as shown in PI. XYI. From a piece ot 

 striped stutf, in which the bright bands are of width equal to 

 about a quarter of the complete period, a photographic negative 

 was taken. The prints from this negative are made with two 

 exposures, between which the paper is slightly rotated. In 

 this way two systems of bands are impressed, crossing one 

 another at a small angle. 



The fact that the two concentric ring-systems cross one 

 another proves of course that they cannot both be strictly 

 circular. Complete circularity in all cases would require a 

 wave-surface in the form of two concentric spheres, and 

 such is not the character of optical double refraction. An 

 exception would occur in the case of a uniaxal crystal cut- 

 perpendicularly to the axis,, which would then be an axis of 

 symmetry for the whole phenomenon. Another comparatively 

 simple case would arise if the surfaces of the plate were 

 parallel to the axis of a uniaxal crystal. In general one at 

 least of the ring- systems would be elliptical as in the obser- 

 vations ; and it would seem that the systems need not even 

 be concentric. A more minute theoretical examination might 

 be of some interest, especially in comparison with observations 

 upon a plate whose optical constants were known. 



Terling Place, Witliam. 

 Oct. 10, 190(3. 



* Wright, Amer. Joum. Sci. xiv. p. 169(1877). 



