Sir D. Brewster on the Colours of Mixed Plates. 193 



As the colours therefore are produced solely by the edges 

 of the cavities, their intensity must, ctjsteris 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 phaenoraena are peculiarly splendid and we are en- 

 abled 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 phaenomena appear. When the 

 vision first becomes indistinct both the direct and the comple- 

 mentary colours appear at the same time, specks of the com- 

 ■plementary red alternating with brighter specks of the direct 

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

 cavities, the complementary specks become less and less visi- 

 ble, and we see only the direct green light. 



In order to study these phaenomena by observing the ac- 

 tion 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 im- 

 mersing thin plates of a solid substance in a fluid of such a re- 

 fractive power, that the thickness of the plates should be vir- 

 tually reduced to the same degree of thinness as the film of 

 albumen between the plates of glass. The new substance de- 

 scribed by Mr. Horner*, and which I shall call nacrite, fur- 

 nished me with the means of performing this experiment. I 

 accordingly inclosed the thinnest films of it between two plates 

 of glass containing balsam of capivi; and I had the satisfac- 

 tion 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 complement- 

 ary 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 phaenomena 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 



• Philosop'.iical Transactions, 183G, p. 49. [or L. and E. Phil. Mag., 

 vol. X. p. 201. 

 Phil. Mag. S. 3. Vol. H. No. 88. Mar. 1839. O 



