Section III, 1922 [283] Trans. R.S.C. 



Investigation of Dispersion by an Interference Method. 



By H. F. Dawes, M.A., Ph.D., 

 Professor of Physics, McMaster University, Toronto. 



(Communicated by J. Patterson, Esq., M.A., F. R.S.C.) 



(Read May Meeting, 1922) 



Synopsis. The paper discusses the distribution of the Inter- 

 ference Fringes of an interferometer system for which the two paths 

 are of unsymmetrical dispersion, for example, a Michelson interfero- 

 meter in one path of which is inserted a plane parallel plate of dis- 

 persive substance such as glass. The spectrum of the interfering 

 light with white illumination shows fringes in the form of a system 

 of closed curves centering on the wave-length for which the number 

 of wave-lengths in the difference of path is a maximum. These are 

 illustrated by spectrograms with sunlight illumination. The numeri- 

 cal results give the dispersive power of the substance to a high degree 

 of precision. 



Introduction 



1. Interference methods are specially suitable for the investiga- 

 tion of dispersion and are in common use for the measurement of the 

 dispersion of a gas. Such methods may also be used to determine the 

 dispersion constants of a substance in the form of a thin film or its 

 equivalent as described, for example, in Mann's Manual of Advanced 

 Optics, pages 35-39, 63, 64. The following paper presents an inter- 

 ferometer method of studying the dispersion of plane parallel plates 

 of considerable thickness with the possibility of measuring the dis- 

 persive power to a high degree of precision. The index of refraction 

 of a prism with accurately worked surfaces may be precisely deter- 

 mined by the Minimum Deviation method. Since, however, the 

 dispersion depends on the differences of the indices in the second 

 decimal place in numbers in the neighbourhood of 1.5 to 1.7 any 

 residual error in the determination of the index by the above ^method 

 becomes relatively some hundred times as important in the determina- 

 tion of the dispersion. The interferometer method, on the other hand, 

 gives the value of the dispersive power with the same degree of pre- 

 cision as the single indçx used in i-educing the observations. The 



