S. R. Williams — An Achromatoscope. 105 



Heretofore we have always had to know the index of refrac- 

 tion of the substance and the thickness of the film in order to 

 find the order of interference, but equation (12) is inde- 

 pendent of both of these as was not the case for equations (1) 

 and (2), hence, as the author* has shown in previous work, we 

 have at our disposal a powerful method for determining the 

 indices of refraction or studying dispersion. f 



(b) Projection of Newton's Mings on the Slit of a Spectrometer. 



If on the other hand, we should project by reflection the 

 image of a set of Newton's rings on the slit of a spectrometer 

 in such a way that the rings were bisected by the slit, then the 

 phenomenon observed in the resulting spectrum is a bird's-eye 

 view at once of all of the successive changes which occurred 

 when the two surfaces were mechanically separated. For that 

 portion of the slit covered by the center of the rings there will 

 be a dark horizontal band running throughout the spectrum, 

 while above and below this band there will appear other dark 

 bands which are inclined to the dark central band. These 

 dark bands are simply those places in the spectrum where for 

 certain wave lengths there is an odd number of half wave 

 lengths difference in path between the two interfering rays. 

 The result is, that if the space between the optical surfaces be 

 considered wedge-shaped along a direction parallel to the slit 

 and the spectrum is a normal one, then in that spectrum there 

 will be straight interference bands which are more nearly par- 

 allel to the central horizontal band, the nearer the approach to 

 the center of the rings is made ; while for greater distances 

 awa} 7 from the center the bands become more nearly parallel to 

 the slit. An attempt has been made to show this graphically in 

 fig. 3, where at the left of the figure is shown the wedge- 

 shaped film of air (/a = i approximately) from which white 

 light is reflected on to the slit, while the corresponding position 

 of the bands in the spectrum are shown to the right. In fig. 1 

 a photograph of a limited portion of the spectrum is shown 

 with these bands appearing. Figs. 3 and 4 were made with the 

 center of the rings at the lower end of the slit, hence these 

 two cuts may both be thought of as quadrants of what actu- 

 ally exists in a grating spectrum, when the dark center is at 

 the center of the slit. 



If the wedge-shaped film is formed from a dispersive 

 medium then the bands become in a measure dispersion curves, 



*Phys. Eev., vol. xviii, p. 280, 1904 ; xxvii, p. 27, 1908. 



fFeussner, as late as 1906, stated that the determination of indices of 

 refraction by means of interference phenomena depended upon equation (2). 

 Winkelmann, Handbuch der Physik, vol. vi, p. 979. 



