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XXXVI. On the Colours of Mixed Plates. By Sir David 

 Brewster, K.G.H., F.R.S* 



nPHE colours of mixed plates were discovered by Dr, 

 Thomas Youngf, and described in the Philosophical 

 Transactions for 1802. He produced them by interposing 

 small portions of water, or butter, or tallow between two 

 plates of glass, or two object glasses pressed together so as to 

 give the ordinary colours of thin plates. In this way portions 

 or cavities of air were surrounded with water, butter, or tal- 

 low ; and on looking through this combination of media he 

 saw fringes or rings of colour six times larger than those of 

 thin plates that would have been produced had air alone been 

 interposed between the glasses. These fringes or rings of 

 colour were seen by the direct light of a candle, and began 

 from a white centre like those produced by transmission; but 

 on the dark space next the edge of the plate, Dr. Young ob- 

 served another set of fringes or rings, complementary to the 

 first, and beginning from a black centre like those produced 

 by reflection. This last set of colours was always brighter 

 than the first. 



The following is Dr. Young's explanation of these two 

 series of colours. 



" In order to understand," says he, " this circumstance, 

 we must consider that where a dark object is placed behind 

 the glasses, the whole of the light which comes to the eye is 

 either I'efracted through the edges of the drops, or reflected 

 from the internal surface; while the light which passes 

 through those parts which are on the side opposite to the 

 dark object consists of rays refracted as before through the 

 edges, or simply passing through the fluid. The respective 

 combinations of these portions of light exhibit a series 

 of colours of different orders, since the internal reflection 

 modifies the interference of the rays on the dark side of 

 the object, in the same manner as in the common colours of 

 thin plates seen by reflection. When no dark object is near, 

 both these series of colours are produced at once ; and since 

 they are always of an opposite nature at any given thickness 

 of a plate, they neutralize each other and constitute white 

 light;." 



• From the Philosophical Transactions for 1838, p. 73. 



t Since this paper was written I find tiiat this class of colours was tlis- 

 coveretl by M. Mazeas, and that his experiments were repeated and varied 

 by M. Dutour, 



J Philosophical Transactions, 1802. Dr. Young republished the same 

 explanation of mixed j)lutes in 1807 in his Elements of'Natural Philosophy. 

 Sec vol. i. pp. 470, 787 ; vol. ii. pp. 035, 680. 



