192 Sir D. Brewster on the Colours of Mixed Plates. 



In so far as I know, these observations have not been re- 

 peated by any other philosopher ; and subsequent authors 

 have only copied Dr. Young's description of the phasnomena 

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

 subject I never doubted the accuracy or the generality of the 

 i-esults obtained by so distinguished a philosopher. I was 

 induced to study the phaenomena of mixed plates as auxihary 

 to a more general inquiry ; and having observed new phaeno- 

 mena of colour in mineral bodies, which have the same origin 

 as those of mixed plates, and which lead to conclusions dif- 

 ferent from those of Dr. Young, I am anxious that they 

 should be described in the same work wMch contains his ori- 

 ginal observations. 



Having experienced considerable difficulty in obtaining 

 satisfactory specimens of the colours of mi^ed plates by using 

 the substances employed by Dr. Young, I sought for a me- 

 thod of producing them which should be at once easy and 

 infallible in its effects. With this view I tried transparent 

 soap, and whipped cream, which gave tolerably good results ; 

 but I obtained the best effects by using the white of an egg 

 beat up into froth. To obtain a proper film of this substance 

 I place a small quantity between the two glasses, and having 

 pressed it out into a film I separate the glasses, and by hold- 

 ing them near the fire I drive off a little of the superfluous 

 moisture. The two glasses are again placed in contact, and 

 when pressed together so as to produce the coloured fringes 

 or rings, they are then kept in their place either by screws or 

 by wax, and may be preserved for any length of time. 



If we now examine with a magnifier of small power the 

 thin film of albumen, we shall find that it contains thousands 

 of cavities exactly resembling the strata of cavities which I 

 have described as occurring in topaz, quartz, sulphate of lime 

 and other minerals* ; and if we look through the film at the 

 margin of the flame of a candle, we shall perceive the two 

 sets of colours described by Dr. Young, the one upon the lu- 

 minous edge of the flame, and the other on the dark space 

 contiguous to it. The first we shall call the direct, and the 

 second, which are always the brightest, the complementary 

 frijiges. 



If we apply a higher magnifying power to the albuminous 

 films, and bring the edge of one of the cavities to the margin 

 of the flame, we shall perceive that both the direct and the 

 complementary colours are formed at the very edge, the com- 

 plementary ones appearing just when the direct ones have 

 disappeared, by the withdrawal of the edge from the flame. 

 * Edin. Trans., vol. x. Part I. p. 407. 



