The London, Edinburgh and Dublin philosophical magazine and journal of science . air-film and D + ^ the thickness of the other, and A», +uX-bm ^m+b the wave-lengths extinguished by interference re-spectively in die thicker and thinner of the adjacent air-films,the indices of the \s giving the order of the bands, then wehave 2 (D + d)= nX.^ = n + 1 X,l+1 = &c. 2D = mVm=m+ lX/m+i = &c., or from the three following equations which correspond tothree interference-bands, 2D=n, from which we have a ^n — A, n X-h + 1 A«— A.«-fl 2 ^n+] (1) Phil. Mag. S. G. Vol. 4. Xo. 24. Dec. 11)02. 2X 658 Prof. J. P
The London, Edinburgh and Dublin philosophical magazine and journal of science . air-film and D + ^ the thickness of the other, and A», +uX-bm ^m+b the wave-lengths extinguished by interference re-spectively in die thicker and thinner of the adjacent air-films,the indices of the \s giving the order of the bands, then wehave 2 (D + d)= nX.^ = n + 1 X,l+1 = &c. 2D = mVm=m+ lX/m+i = &c., or from the three following equations which correspond tothree interference-bands, 2D=n, from which we have a ^n — A, n X-h + 1 A«— A.«-fl 2 ^n+] (1) Phil. Mag. S. G. Vol. 4. Xo. 24. Dec. 11)02. 2X 658 Prof. J. Patterson on the Electrical That is, the number of band-widths Xn is displaced towards 2 which the red end of the spectrum, gives a multiple ofis the thickness required. The displacement of one set of bands relatively to theother was measured with the spectrometer. The optical glasswas placed in front of the slit of the collimator (tig. 6). Fig. The line of junction of the two laminae was placed so as to beperpendicular to the slit. The light from a Welsbach entered the collimator throughan opening A, and part of it was reflected by the glass mirrorC through the collimator-slit, and fell at perpendicular inci-dence on the air-film. It was then reflected back throughthe collimator and glass prism D and viewed by the telescopeE. Two sets of dark bands were thus obtained in the spec-trum, one of which was displaced relatively to the other. Bymeasuring the angular width of each band and the displace-ment of the corresponding band in the other set, the thicknessof the metallic film was obtained from formula 1. Thespectrometer was calibrated by means of known lines and acurve was drawn connecting wave-lengths and the angles of Fig. 7.
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