Intelligence and Miscellaneous Articles. 255 



reflection was in air. There also the results of the earlier investi- 

 gations by Airy* and Giant of Newton's rings between lenses and 

 plane mirrors of various materials are treated of, as well as the 

 influence exerted upon experiments of this sort by condensed vapour 

 or gas films on the reflecting surfaces. I would seek in such im- 

 purities the reason of the difference between my measurements and 

 those of other observers. 



Before the appearance of the communications in question, I had 

 pointed outj "that the amplitude and phase of light in passing 

 through thin layers of metal are changed simultaneously, the change 

 depending on the thickness of the metal," and also " that the change 

 of phase in refraction must vary also with the position of the plane 

 of polarization." At the same time, I showed how, independently 

 of the sources of error of the formerly usual methods, a thin lamella 

 of metal displaces interference-streaks as if the refraction-index 

 in the interior of the metal were < 1§. On the same occasion it 

 was explained how far it is justifiable to assume an acceleration o£ 

 the phase when light passes through thin, transparent layers of 

 metal, which M. Potier seeks to establish on theoretical grounds. 



"When a lens is pressed upon a glass plate partially covered with 

 a thin layer of metal, Newton's colour-rings upon the metal appear 

 displaced towards those upon the glass plate. The thickness of 

 the metal cannot be calculated from this displacement, unless, as is 

 correctly remarked in the communication mentioned, the changes 

 of phase with reflection at the surfaces of the metal and the glass, 

 or the difference between the two, be known. 



Hence I have never made use of this method for determining 

 the thickness, but have pressed the lens upon the uncovered part 

 of the glass plate until the lower surface of the lens touched the 

 margin of the sharply bordered layer of metal. Contact was shown 

 by the distortion of the colour-rings on the glass. The colour 

 which was exhibited by the layer of air of the same thickness as 

 the metal on the glass plate gave then the thickness of the latter |[. 

 It is at once seen that in this determination the knowledge of the 

 change of phase with normal reflection is not requisite ; and there- 

 fore ignorance of it could not occasion any error in the rest of the 

 conclusions deduced by me. 



Finally, I have also shown how, in reflection from the same 

 metal, the principal angle of incidence and principal azimuth, as 

 well as the qualities generally of the reflected light, depend on the 

 nature of the transparent medium in which the reflection takes 

 place IT, and to what depth light of different colour and plane of 

 polarization penetrates the metal ; so that, in relation to this also, 

 M. Potier's remarks cannot be said to be new. — Poggendorff's 

 Annalen, vol. cxlviii. pp. 311-316. 



* Carabr. Trans, vol. iv. ; Pogg. Ann. vol. xxvi. p. 123 : 1832. 

 t Absolute Phasenanderungen durch Reflexion : Berlin, 1870. 

 X Poscg. Ann. vol. cxli. p. 191 : 1871. 



§ Ibid. p. 186 : 1871. II Ibid. vol. cxxix. p.^178 : 1866. 



*[ Ibid. vol. cxxviii. p. 547 sq., cxxix. pp. 182ew217 : 1866. 



