SIR DAVID BREWSTER ON THE COLOURS OF MIXED PLATES. 75 



specks of the direct green light. By increasing the distance of the lens from the 

 cavities, the complementary specks become less and less visible, and we see only the 

 direct green light. 



In order to study these phenomena by observing the action of a single edge upon 

 light, and to ascertain the effect of an edge when there were no prismatic edges to 

 refract, and no internal surface to reflect light, I conceived the idea of immersing thin 

 plates of a solid substance in a fluid of such a refractive power, that the thickness of 

 the plates should be virtually reduced to the same degree of thinness as the film of 

 albumen between the plates of glass. The new substance described by Mr. Horner*, 

 and which I shall call nacrite, furnished me with the means of performing this ex- 

 periment, I accordingly inclosed the thinnest films of it between two plates of glass 

 containing balsam of capivi; and I had the satisfaction of observing that the bounding 

 edge of the plate and the fluid produced the identical direct and complementary 

 colours above described. 



The bounding edge which I selected for observation gave a bright green for the 

 direct, and a bright red for the complementary tint. This edge appeared as a narrow 

 distinct black line, exceedingly well defined, and of a uniform breadth like the finest 

 micrometer wire. It consequently obstructed the incident light and produced the 

 phenomena of diffracted fringes. These fringes, however, were modified by the pe- 

 culiar circumstances under which they were produced, and exhibited in their tints 

 both the direct and complementary colours under consideration. 



When the diffracted fringes are viewed in candle-light by a lens placed at a greater 

 distance from the diffracting edge than its principal focus, the middle of the system 

 of fringes corresponding to the diffracted shadow of a fibre is occupied with the direct 

 tint, which we shall suppose to be green. ; and on each side of this green shadow, as 

 we may call it, we observe very faintly the complementary red tinging what are called 

 the two first exterior fringes. This tinge of red is strongest in the first fringe within 

 the solid edge, or within the green shadow, while it is yellowish in the first fringe 

 without the green shadow. These effects are inverted if we place the lens nearer to 

 the edge than its principal focus. 



The phenomena now described appear more distinct if we take an extremely nar- 

 row piece of nacrite, having its two edges nearly in contact, and transmitting only a 

 narrow line of light. In this case the two red fringes within the solid edge unite their 

 tints, and become a bright red ; and in like manner if we place the lens nearer the 

 solid edges than its principal focus, the two yellow fringes will unite their tints, and 

 become a brighter yellow band. In this last case, when the two bounding edges are 

 still nearer each other, the united fringes, in place of being yellow, will be green, or 

 the same as the direct colour. 



If we bring the edges of two pieces of nacrite of equal thickness very near each 

 other, having, as formerly, green for the direct, and red for the complemaitary colour, 



* Philosophical Transactions, 1836, p. 49. 

 l2 



