REPORT ON PHYSICAL OPTICS. ;347 



the less the inclination of the plates *. Tlie colours in the first 

 lateral image are producedby the interference of the pencils which 

 have undergone two reflexions, — one of them being reflected 

 internally by the first plate, and externally by the second, while 

 the other is reflected internally 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 f. The 

 interfering rays in this case appear to be those which have un- 

 dergone 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 concenti-ic rings is formed in this 

 case, the dimensions of which are greater than those of either of 

 the primary 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 these are 

 equal J. 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 un- 

 der 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 



* Ellin. Trans., vol. vii. p. -13.5. 



t Elemens de P/ii/,siquc, toni. ii. p. 478. 



: P/iil. Irans. 1815, p. 161. 



