76 REPORT ON PHYSICAL OPTICS. 



nally by the second, and externally by the first. The routes of 

 these portions differ only by reason of the different inclinations 

 at which they traverse the intervals of the surfaces. M. Pouillet 

 has observed a phenomenon of the same kind, when a thick plate 

 of glass is placed above a metallic mirror, and in a direction nearly 

 parallel to its surface.* The interfering rays in this case appear 

 to be those which have undergone two reflexions within the plate, 

 and one at the surface of the mirror, the reflexion from the mirror 

 preceding the others in the case of one pencil, and following them 

 for the other. The routes of two such pencils will slightly differ, 

 owing to the different obliquity under which they traverse the 

 plate. 



The remarkable phenomena observed by Mr. Knox, when a 

 double-convex lens was combined with two plane glasses, one 

 adjacent to each surface, have been explained by Young on the 

 same principles. In addition to the rings exhibited by each plate 

 of air, a third system of concentric rings is formed in this case, the 

 dimensions of which are greater than those of either of the pri- 

 mary systems. The diameters of these rings increase indefinitely 

 as those of the primary systems approach to equality ; until finally 

 the circles become straight lines when they are equal, f It is 

 easily seen, in fact, that each ring is the locus of the points for 

 which the difference of the thicknesses of the two plates of air 

 is constant ; and that this locus is a circle, whose diameter will 

 depend on the curvatures of the surfaces, and on the interval of 

 the centres of the two primary systems. The fringes formed by 

 " double plates " have been observed under another form by Mr. 

 Talbot, when two films of thin blown glass were superposed. 



The "colours of thick plates" are perhaps of too unusual 

 occurrence to entitle them to be studied as a separate class of 

 optical phenomena; the attention which they have received is 

 owing to the investigations of Newton. In the experiment of 

 Newton a beam of light is admitted through a small aperture, 

 and received on a concavo-convex mirror with parallel surfaces, 

 the second of which is silvered. When a screen of white paper is 

 then held at the centre of the mirror, having a hole in the middle 

 to allow the beam to pass and repass, a set of broad coloured rings 



* Element de Physique, torn. ii. p. 478. 

 t Phil. Trans., 1815, p. 161. 



