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LXIL On the Cause of the ■ Coloured Bands 9 observed by A. R. 

 By A Correspondent. 



T^HE coloured bands described by A. R. in the last number 

 A of this Journal, p. 363, are exactly the same as those disco- 

 vered by Sir David Brewster and described in the Edinburgh 

 Philosophical Transactions, vol. vii. page 435. Sir John Her- 

 schel, in his Treatise on Light, page 475, 476, has given a 

 minute account of these bands as " affording an excellent 

 illustration of the laws of periodicity observed by the rays 

 of light in their progress, whether, as in the Newtonian doc- 

 trine, we consider them as subjected to alternate fits of easy 

 reflection and transmission, or, as in the undulatory hypothesis, 

 as passing through a series of phases of alternately direct and 

 retrograde motions in the particles of aether in whose vibra- 

 tions they consist." 



The bands under consideration are produced entirely by 

 the plates of parallel glass between which A. R. had placed 

 his convex lens, and are dependent upon the inclination of 

 these plates, to the common section of which they are parallel. 

 Since the publication of his memoir Sir David Brewster has 

 observed the same fringes stretching with singular brilliancy 

 across the fourth and sixth images formed by total reflection 

 from the posterior surfaces of two plates of common mirror 

 glass inclosing water. 



In the work already referred to, Sir John Herschel has 

 given a perspicuous explanation of these phaenomena in their 

 general details ; and he has adverted also to another series of 

 coloured fringes coexisting with the first series, which Sir 

 David Brewster describes " as far surpassing in precision of 

 outline and in richness of colouring every analogous phaeno- 

 menon which he had seen." 



" By intercepting," says Sir John, " the principal trans- 

 mitted beams in the direct image, and receiving only those 

 portions of the rays going to form it whose curves are as in fig. 

 140, Dr. Brewster succeeded in rendering visible a set of co- 

 loured fringes, which in general are diluted and concealed in 

 the overpowering light of the direct beam. They originate, 

 evidently, in the interference of those two rays whose courses 

 are each represented by 4 / + 1, and would therefore be 

 strictly equal if the plates were exactly parallel. Their theory, 

 after what has been said, will be obvious on inspection of the 

 figure, as well as those of all the rest of the systems of fringes 

 described in that highly curious and interesting memoir." 

 (Herschel on Light, p. 476, § 694.) 

 Nov. 11th, 1835. 



