76 SIR DAVID BREVVSTEa ON THE COLOURS OF MIXED PLATES. 



the space between the edges, or between the green bands, will be faint red when the 

 lens is nearer the edges than its principal focus, and yellow when it is further from 

 them ; but if the edges are brought still nearer, the faint red will become brighter, 

 and the united green bands will take the place of the yellow one. 



Let us now return to our plate of nacrite with a single edge, having green and red 

 for the two tints; and let us always suppose that the lens is adjusted to observe the 

 diffracted fringes, that is, that the lens is placed at a greater distance from the dif- 

 fracting edge than its principal focus. We shall also suppose that the light of the 

 sun passing through a narrow aperture parallel to the diffracting edge is substituted 

 for the light of a candle. Under these circumstances the central part of the system 

 of fringes seen by light incident perpendicularly, consists of blue*, green, and yellow 

 light, constituting, as it were, the shadow of the edge, the blue light being on the same/ 

 side as the plate of nacrite, and the yellow rays encroaching upon the exterior faint red 

 band already described, the other red band next the blue being more distinctly seen. 

 If we now incline the incident ray to the plate of nacrite more than 90°, the faint red 

 band next the yellow gradually becomes brighter, while the other bands become 

 fainter ; and at the boundary of light and darkness all the other bands disappear ex- 

 cept this red one, which is the complementary colour to i\\Q green, (produced by the 

 union of the blue, green and yellow bands,) and the colour which is seen upon the dark 

 space next the edge of the flame, as described by Dr. Young. If we, on the other 

 hand, incline the incident ray in an opposite direction, so that it forms with the 

 plane of the plate a less angle than 90°, the red band next the blue will now become 

 brighter ; and at the boundary of light and darkness, when all the other bands have 

 disappeared, the red band will afford the complementary colour to the green. 



As the edge of the plate of nacrite is rough and unpolished, and accurately per- 

 pendicular to the parallel faces, there are no reflected nor refracted pencils, whose 

 combinations with one another, or with the direct rays, can be employed to account 

 for the complementary colours. The phenomena of mixed plates, indeed, are cases 

 of diffraction when the light is obstructed by the edge of very thin transparent plates 

 placed in a medium of different refractive power. If the plate were opake the fringes 

 would be exactly those which have been so often described, and explained by the prin- 

 ciple of interference. But owing to the transparency of the plate, fringes are pro- 

 duced within its shadow ; and owing to the thinness of the plate the light transmitted 

 through it and retarded, interferes with the partial waves which pass through the plate 

 and with those which pass beyond the diffracting edge with undiminished velocity, 

 and modifies the usual system of fringes in the manner which we have described. 



As the plate of nacrite diminishes in thickness, or as the fluid in which it is im- 

 mersed approaches to it in refractive density, the central coloured bands, whose union 

 constitutes the direct tint, will diminish in number, and descending gradually in the 

 scale will finally disappear when the retardation produced by the plate does not per- 



* Owing to the small quantity of blue rays in candle-light the blue almost disappears in it. 



