/S. R. Williams — An Aohromatoscope. 



107 



be said however, that for interference dispersion curves in 

 which points of inflection do occur there can be little doubt 

 but what that substance shows anomalous dispersion. In tig. 5 

 is shown a rough drawing of interference dispersion curves 

 from a thin film of a concentrated solution of alcohol and 

 fuchsine in which points of inflection do occur, Some excel- 

 lent photographs showing these inflections in interference dis- 

 persion curves may also be seen in an article by W. H. Julius," 

 who used a Jamin interferometer for getting the interference 



Fig. 5. Interference dispersion curves from a thin film of concentrated 

 alcohol-fucksine solution. Satisfactory cuts could not be made from original 

 photographs. 



effects. Frickef and Puccianti^: have also studied the disper- 

 sion of absorbing media by methods which suggest that given 

 in this paper. This method also recalls the method of crossed 

 prisms used by Newton§ and the crossed prism and grating 

 method of Wood.|| 



As stated in the beginning of (b) when the image of the 

 rings of Newton is projected on the slit of the spectrometer 

 a view is had at once of all the steps which occur when the 

 optical surfaces are slowly separated as in (a). In projecting 

 the rings of Newton on the slit of the spectrometer care must 

 be taken that they are centrally placed, otherwise distorted 

 interference bands will occur. A wedge-shaped film, of course, 

 obviates this difficulty. 



Another way of showing this relation is to use a long narrow 

 slit, the length of the spectrum, and move the same parallel to 

 itself and to the central band. Whatever appears in the nar- 

 row slit will be some step which has occurred when the sur- 

 faces were separated in (a). If along any Fraunhofer line one 

 counts the number of interference bands between this slit and 

 the central band, the number of bands will give the order of 

 interference for the thickness of film which is located at a dis- 



* Julius, Astrophys. Jour., July, 1914. 

 fFricke, Drude's Annal., xvi, p. 865-889, 1905. 

 {Puccianti, Nuovo Cim., ii, p. 257-264, 1901. 

 § Newton, Opticks, book 1, Prop. 2, theorem 2. 

 I Wood, Physical Optics, p. 115, ed. 2. 



