120 



COLORS OF THICK PLATES. 



When llie system of lenses det-cribed above is held between the eye and the 

 li;^lit, another .•system of ruigs makes its appearance, which is formed by the 

 transmitted light. In this case the tints are much feebler, being diluted by the 

 intermixture (;f a great deal of whit3 liglit, which, as we shall see hereafter, lias 

 nothing to do with their formation. Of these it is remarkable that the diame- 

 ters of the briglit rings correspond with those of tlie dark rings seen by reflec- 

 tion. Tims the thicknesses at which the bright rings by transmitted light 

 appear form a scries corresponding witli the progression of even numbers, 0, 2, 

 4, G, &c. ; and the thicknesses at which the intervening dark rings are seen cor- 

 respond to the progression 1, 3, 5, 7, &c. Also the tints reflected and trans- 

 mittt'd at any given point are complementary to each other, or are such as, 

 united, produce white. 



The measurements above given are those which correspond to rings formed 

 by light perpendicularly incident upon the thin lamina. But wlien the rings 

 arc observed obliquely, their diameters are rapidly enlarged with increase of 

 obliquity. Sir Isaac Newton ascertained the law of this increase to be this : 

 that the squares of the diameters arc inversely as the cosines of incidence. 

 When the incidence exceeded GO^, it appeared to him that this law uo longer 

 held good; and this conclusion, which, up to a recent period, had not been 

 invalidated, has formed a serious diiliculty in the way of any tlieory of light. 

 Ilecent experiments, however, made by Messi's. Provostayc and Desains, with 

 monocliromatic light, and with special arrangements to eliminate the sources of 

 error in m(;asurement which must liavi; vitiated Newton's results at high inci- 

 dences, have fully established the; universality of the law. Their measurements 

 extended to the forty-third ring, and to the great incidence of 86° 14', beyond 

 which the rings were no longer discernible. 



Colors resembling those of thin plates may be ju-oduced also, in various modes, 

 by means of thick plates. Sir Isaac Newton employed, in an interesting expcjri- 

 ment of this kind, a spherical glass mirror, with truly concentric surfaces, silvered 

 on the back. A very small bcsam of light (about one 

 twenty-fifLh of an inch in diameter) having been intro- 

 duced int(5 a dark room, he received it on this mirror in 

 such a manner as to reflect it back loward the aperture. 

 At the centre of curvature of the mirror he placed a white 

 card pierced, in order to allow the light to pass, with a 

 very small orilice. Around this orilice he saw a seri(js of 

 rings resembling those of thin plates. When the light 

 was homog(;neous, the rings were alternately bright and dark as in the other case. 

 The diameters were also observed to follow similar laws. As both surfaces of 

 the mirror are concerned in producing these rings, and as, at the flrst surface, 

 it is the irregular or scattered reflection only which is necessary to the eficct, 

 the experiment succeeds best with a mirror in which this surface is not highly 

 polished. Instead of the perforated card, a lamina of mica, or of slightly tar- 

 nished glass, may be employed to receive the rings. 



When light is transmitted through or reflected by a pair of thick plates of 

 homogeneous glass, with parallel plane surfaces, 

 and placed parallel to each other, colors may ap- 

 pear, if the difference of thickness of the two ])lates 

 is comparable to the absolute thickness at which 

 sucli colors arc produced by thin plates. The 

 ligurc! shows the arrangement. Dr. 13r(nvster pro- 

 duced th(! same effects with a pair of plates of 

 equal thickness, by inclining one of them so that 

 lh(! path of the rays witliin it sliould be slightly 

 longer than M'ithin the other. There; is some sim 

 ilarity between the first of these classes of phenomena and those of diffraction. 

 The second have a nearer analogy to the colors of Newton's rings. 



M 



FlK. 8. 



