THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MAY 1855. 



XLVII. On the Interference of Light near a Caustic, and the Phe- 

 nomena of the Rainbow. By Professor Potter, A.M.* 



AT the meeting of the British Association for the Advance- 

 ment of Science at Cambridge in 1833, I exhibited the 

 experiment of the interference of light near a caustic, which I 

 had a short time before discovered when examining the circle of 

 aberration of a spherical mirror for a reflecting microscope. This 

 circle of aberration was formed by using a luminous point, which 

 was an image of the sun given by a small globule of mercury, 

 for an object of which the image produced by the spherical 

 mirror was to be examined. In place of the circle of aberration 

 being a disc brightest at the edge and shading away gradually 

 towards the centre, as we should expect from geometrical optics, 

 it appeared a magnificent phenomenon of the interference of 

 light, consisting of brilliant and black rings coloured on their 

 edges. 



The outer ring, instead of being brightest at its outer edge, 

 as we should have expected from the undulatory theory, since 

 the rays had travelled over equal spaces, shaded away gradually. 

 I concluded that I had fallen upon a confirmation of what I had 

 before found in experiments with the two mirrors slightly in- 

 clined, namely, that the central band of interference, where the 

 rays have travelled over equal distances, was in normal circum- 

 stances a black band and not a bright one, as it had been asserted 

 to be by the late M. Aragof and Professor Airy J, in accordance 

 with the undulatory theory of light. 



* Communicated by the Author. 



t See Annates da Chimie et da Physique for 1819, vol. xi. p. 12. 



X Tract on the Undulatory Theory of Optics, articles 51 and 54. 



l'lal. Mag. S. 1. Vol. 9. No. GO. May 1855. Y 



