[148] 



PROFESSOR STOKES, ON THE 



indeed, but sufficient to establish the identity of the effect. It is unnecessary here to discuss 

 the theoretical views of the Duke de Chaulnes, since the progress of optical science has since 

 led to a complete explanation of the formation of the rings. 



The colours of thick plates were first explained on the undulatory theory by Dr Young*, 

 by whom they were attributed to the interference of two streams of light, of which one is 

 scattered on entering the glass, and then regularly reflected and refracted, and the other 

 regularly refracted and reflected, and then scattered on its return through the first surface. 

 Dr Young's explanation is however excessively brief; and he has rather pointed out the 

 application of the grand and newly-discovered principle of interference to the explanation of 

 the phenomenon, than followed the subject into any of its details. At the same time, it 

 appears evident, from an attentive perusal of what he has written, that at least the broad 

 outlines of the complete explanation were clearly present to his mind. 



In the course of a paper entitled " Experiments for investigating the cause of the coloured 

 concentric rings, discovered by Sir Isaac Newton, between two object-glasses laid upon one 

 another," Sir William Herschel mentions an experiment in which rings of the nature of those 

 of thick plates were produced by merely strewing hair-powder in the air in front of a metallic 

 speculum placed as the mirror in Newton's experiment-}-. The result of this experiment was 

 justly regarded by Herschel as inexplicable on the theory of fits. It may here be remarked 

 that it is in perfect accordance with the theory of undulations. 



In the Annates de Chimie et de Physique\, will be found a report by Ampere and 

 Poisson on a memoir by M. Pouillet, containing some experiments on the rings. The experi- 

 ments were mostly the same as those of the Duke de Chaulnes, but accompanied by measures. 

 M. Pouillet found that the rings were produced by placing in front of a metallic speculum an 

 opaque screen containing an aperture of any form. In this case the rings were round, what- 

 ever might be the form of the aperture. The experiments are mentioned by M. Pouillet in 

 his Eletnens de Physique^. 



A complete explanation of the rings, according to the theory of undulations, has been given 

 by Sir John Herschel in his treatise on Light ||. The rings are supposed to be formed in 

 Newton's manner with a glass mirror, the luminous point being situated in the axis. Having 

 investigated the elementary system of rings which would be produced by the two streams 

 scattered in passing and repassing at the point of the first surface where it is cut by the axis, 

 Sir John Herschel shews that if the surfaces be supposed to be a pair of concentric spheres, 

 having the luminous point in their centre, the elementary systems corresponding to the several 

 elements of the first surface will be superposed, in such a manner that a distinct system would 

 be thrown on a screen held at the distance of the luminous point. The laws of the rings 

 resulting from theory are precisely those which had been discovered by Newton, and the calcu- 

 lated magnitudes were found to agree almost exactly with Newton's measures. 



A set of coloured bands have since been observed by Dr Whewell, which are formed when 

 the image of a candle held near the eye is viewed by reflexion in a plane mirror of quick- 



* On the Theory of Light and Colours. Philosophical 

 Transactions for 1802, p. 41. 



f Philosophical Transactions for 1807, p. 231. 



t Tom. i. (1816) p. 87. 



§ Tom. ii. p. 476. 



|| Encyclopaedia Metropolitana, Arts. 676, &c. 



