74 SIR DAVID PUEWSTER ON THE COLOURS OF MIXED PLATES. 



never doubted the accuracy or the generality of the results obtained by so distinguished 

 a philosopher. I was induced to study the phenomena of mixed plates as auxiliary 

 to a more general inquiry ; and having observed new phenomena of colour in mineral 

 bodies, which have the same origin as those of mixed plates, and which lead to con- 

 clusions different from those of Dr. Young, I am anxious that they should be de- 

 scribed in the same work which contains his original observations. 



Having experienced considerable difficulty in obtaining satisfactory specimens of 

 the colours of mixed plates by using the substances employed by Dr. Young, I sought 

 for a method 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 effect by using the white of an egg 

 beat up into froth. To obtain a proper film of this substance I place a small quan- 

 tity between the two glasses, and having pressed it out into a film I separate the 

 glasses, and by holding 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 ca- 

 vities 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 luminous 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 fringes. 



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 complementary 

 ones appearing just when the direct ones have disappeared, by the withdrawal of the 

 edge from the flame. 



As the colours therefore are produced solely by the edges of the cavities, their in- 

 tensity must, cceteris paribus, depend on the smallness of the cavities, or the number 

 of edges which occur in a given space. When we succeed in forming an uniform 

 film in which the cavities are like a number of minute points, the phenomena are 

 peculiarly splendid and we are enabled to study them with greater facility. When the 

 edges of these cavities are seen by an achromatic microscope, and in direct light, 

 neither the direct nor the complementary colours are visible ; but if we gradually 

 withdraw the lens from the cavities a series of beautiful phenomena appear. When 

 the vision first becomes indistinct both the direct and the complementary colours 

 appear at the same time, specks of the complementary red alternating with brighter 



* Edin. Trans. voL x. Part I. 407, 



