208 Messrs. E. Edser and C. P. Butler on a Simple 



one band falls on the slit, the resulting spectrum will be 

 crossed by vertical black bands, varying in number and 

 breadth with the order of the interference-band from which 

 the spectrum is derived*. If the interference-fringes are 

 displaced across the slit the black bands in the spectrum will 

 become finer and more numerous as the central interference- 

 fringe recedes from the slit. Even when the coloured fringes 

 have become invisible at the position of the slit owing to the 

 high relative retardation of the interfering pencils, the bands 

 in the spectrum remain quite distinct, becoming indistinguish- 

 able only when so fine that the resolving and dispersive 

 powers of the spectroscope are insufficient to separate them. 



In our earlier experiments we focussed the image of an 

 air-film, contained between two plane and parallel glass sur- 

 faces, on the slit of the spectrometer j\ The image should be 

 obtained by means of light reflected from the film, the 

 spectrum bands obtained when transmitted light is used being- 

 very faint. This method, which is theoretically the most 

 perfect, has the disadvantage that a somewhat careful ad- 

 justment is necessary in order to insure good results. We 

 have therefore sought for some simpler method. It is un- 

 necessary here to detail the various methods which we have 

 successively tried ; it will suffice to describe the arrangement 

 ultimately adopted as being the simplest, whilst complying 

 sufficiently closely with the ideal conditions to insure trust- 

 worthy results. 



Let us suppose that a transparent parallel-sided film of 

 thickness d is placed immediately against the slit of the 

 spectroscope and illumined with white light. Owing to the 

 interference of the ray directly transmitted and that twice 

 internally reflected within the film there will be bright bands 

 in the spectrum separated by darker intervals, the wave- 

 lengths X , A 1? \ 2 , . . . \ r , . . . \ m corresponding to the bright 

 bands being given by the equations 



2/id=n\ =(7i+l)\ 1 =(tt+ 2)\ 2 = . . . = (n + r)\ r = {n+m)\ n 

 where n is the refractive index (supposed independent of the 

 wave-length) of the substance of the film, whilst n may be 

 any integral number. 



If \ and \ m are known n can be determined from the 

 equation 



» = T 7~> (1) 



/VQ-— A. 



* Fizeau and Foucault, Ann. de Chim. et de Phys. 3rd series, torn. xxvi. 

 p. 138 (1849) ; Comptes Rendus, Nov. 24, 1845. 



t An air-film has been used somewhat similarly by Rubens in order to 

 calibrate a prism for infra-red light. Wied. Ann. vol. xlv. (1892) p. 238. 



