C. Barus — Displacement Interferometer. 109 



Art. XYI. — A Displacement Interferometer Adapted for 

 High Temperature Measurement, Adiabatic Transforma- 

 tions of a Gas, etc. ;* by C. Barus. 



1. Elliptic Interferences. — Interferometry by displacement 

 has an advantage inasmuch as the observer never loses the 

 ellipses, even when the displacement is sudden. Their center 

 may always be brought back again to a given spectrum line, by 

 the micrometer. Moreover, since 



JST C = 6/xcos B =- C -~ = e cos B (/x + 2 B / cos 2 B) 



cos jl\i aA. 



where JV C is the reduced micrometer reading, e the thickness, 

 fx the index of refraction of the glass plate of the grating, for 

 the wave length X, R the angle of refraction, and where 

 fx — A + B/X"*, the sensitiveness, may be regulated by decreas- 

 ing the thickness of the grating, <?, by aid of a compensator of 

 thickness e\ for the virtual thickness is now e—e'. Hence, 

 since for radial motion the sensitiveness per fringe across any 

 given Fraunhofer line is 



dJV/dn = \/2 



this may be combined with the shift of ellipses controlled by 

 JY C , in any ratio. The limit of this procedure is conditioned 

 by the size of the ellipses or the available, size of the field of 

 the telescope, since w T hen e—e' approaches zero the ellipses 

 become enormous. 



Furthermore it has been shownf that the quantity dfi/dX 

 occurring in the value of JY C may be computed preliminarily 

 from observations of AJV C = JV C — j¥ c ', between definite Fraun- 

 hofer lines, particularly when the angles of incidence 7, and 

 of refraction It, are small. In such a case the constant SeB = /3 

 nearly, where (/== 0) 



Ap=B(\/\*-l/\\), A2V c = i 3(lA 2 _l/V E ). 



Finally if AJV is the motion of the micrometer to bring the 

 center of ellipses back to a given line 



AJST= (p-I) e' 

 where e r is the thickness of the compensator. If e r is large, one 

 may expect to distinguish between the indices of refraction of 

 a birefringent crystal, w T hen the source of light is polarized. 

 Again when the arms or the interfering beams of light are 

 long, the refraction of a gas and its relation to temperature and 

 pressure are determinable. 



* Abridged from the Eeport to the Carnegie Institution of Washington. 

 Reprinted by permission. 



f Carnegie Publications, No. 149, 1911, chap, v, §44. 



