Properties of Thin Metal Films. (357 



fastened with a little soft wax at the edges. When a beam 

 of parallel white light was allowed to fall on this, interference- 



bands were produced by the thin air-film between the two 

 plates. 



As the air-film in one part was thicker than in the other 

 by the thickness of the first film deposited, two sets of inter- 

 ference-bands were produced, one of which was displaced 

 relatively to the other. By measuring the width of each 

 band in the one set and the displacement of the corresponding 

 band in the other set the thickness of the metallic film conld 

 be obtained. If D (fig. 5). is the thickness of the thinner 



Fio-. 5. 



air-film and D + ^ the thickness of the other, and A», \ n +u 

 X-'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 we 

 have 



2 (D + d)= nX.^ = n + 1 X, l+1 = &c. 



2D = mV m =m+ lX / m+ i = &c., 



or from the three following equations which correspond to 

 three interference-bands, 



2D=n\' 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 



