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V. On the Colours of Mixed Plates. By Sir David Brewster, K.G.H. F.R.S. 



Received October 25, — Read December 14, 1837. 



The colours of mixed plates were discovered by Dr. Thomas Young*, 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 tallow; 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 ciindle, and began from a white centre like those produced by 

 ti*ansmission ; but on the dark space next the edge of the plate. Dr. Young observed 

 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 refracted 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 re- 

 flection 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-f-." 



In so far as I know, these observations have not been repeated by any other philo- 

 sopher ; and subsequent authors have only copied Dr. Young's description of the 

 phenomena and acquiesced in his explanation of them. In taking up this subject I 



* Since this paper was written I find that this class of colours was discovered by M. Mazeas, and that his 

 experiments were repeated and varied by M. Dutour. 



t Philosoplucal Transactions, 1802. Dr. Young republished the same explanation of mixed plates in 1807 

 in his Elements of Natural Philosophy. See vol. i. p. 470, 787; vol. ii. 635, 680. 



MDCCCXXXVIII. L 



