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XCIV. The Rectification of the Spectrum in Relation to 

 the Shift of Ellipses in Displacement Interferometry. By 

 G. Barus, Ph.D., LL.D., Hazard Professor of Physics, 

 Brown University, Providence, R.I., U.S.A.* 



1. Introductory. — In my work of last year f the quantity 

 jx — X -W' (/£ index of refraction of glass, X wave-length) 



occurred in the reductions, and in the absence of direct data 

 I temporarily made use of the corresponding quantity com- 

 puted from standard data for a somewhat similar glass. In 

 this way the amount of displacement of the centres of ellipses 

 per cm. of displacement of the micrometer was determined. 

 It must, however, frequently be necessary to find dpjdX from 

 a small piece of glass, and I have therefore tested the following 

 method in which a Kohlrausch total reflectometer slides along 

 the graduated rail of Rowland's adjustment for the grating, 

 as modified for the plate grating by M. Barus and myself J. 



The method is then further modified, in a way that is appli- 

 cable when a glass grating is given for investigation, as in 

 the immediate problem, but not otherwise. It admits of some 

 interesting applications, as for instance to the refraction of 

 the material of film gratings. 



In all cases of solids, the refraction of the more strongly 

 refracting liquid in which the solid is submerged must of 

 course be known for all wave-lengths ; but this is the case 

 for carbon disulphide. If the liquid indices are not known 

 throughout, in terms of wave-length, they may be determined 

 by aid of an air-plate, using virtually the same apparatus and 

 method. I shall give examples of all these measurements 

 below. 



2. Apparatus. — In fig. 1 AA and BB are the two fixed rails 

 of Rowland's adjustment at right angles to each other, R, 

 joining the vertical axes at a andp, is the cross rail, stretching 

 from the slide D which carries the grating g to the slide C 

 which carries the total reflectometer T. Slides C and D move 

 at right angles to each other, and the position x of C is 

 measured along the brass millimetre-scale SS by aid of the 

 vernier v. 



* Communicated by the Author. 



Abridged from the Report to the Carnegie Institution of Washington, 

 U.S.A. 



t Carnegie Publications, No. 149, 191 1, " The Production of Elliptic 

 Interferences in Relation to Interferometry," Phil. Mag. [6] xxi. p. 411 

 (1911). 



X American Journal, xxxi. pp. 85-95 (1911). 



