486 C. Barus — Interferences of Crossed Spectra. 



Art. XXXVI. — On the Interferences of Crossed Spectra and 

 on Trains of Beating Light Waves; by C. Barus.* 



1. Introductory . — If two component spectra from the same 

 source coincide throughout their extent, the elliptic interferences 

 will be spread over the whole surface, provided of course the 

 respective glass and air path differences of the component rays 

 are not too great to interfere with visibility. In the usual 

 method of producing these interferences, where the correspond- 

 ing reflections and transmissions of the two component rays 

 take place at the same points of the same plane surface, the 

 interference pattern is automatically centered, or nearly so. 

 This is not the case when, as in the following experiments, 

 the interfering beams are produced in some other way ; and 

 the problem of centering is one of the chief difficulties 

 involved. But if the four beams are to be treated independ- 

 ently, it is difficult to obviate this annoyance. 



Suppose now that one of the spectra is rotated around an 

 axis normal to both, by a small angle. Will the interferences 

 at once vanish, or is there a limiting angle below which this is 

 not the case? In other words, in how far can one trench 

 with light waves upon the case of musical beats or of 

 interferences not quite of the same wave length ? 



Instead of approaching the question in this form, in which 

 it would be exceedingly difficult experimentally, I have 

 divided it into two component parts. Let one of the spectra 

 be rotated 180° degrees around a longitudinal axis, parallel to 

 the red-violet length of the spectrum and normal to the 

 Fraunhofer lines. In such a case, interference should be 

 possible only along the infinitely thin longitudinal axis of 

 rotation to which both spectra are symmetrical, one being the 

 mirror image of the other. One would not expect these 

 interferences to be visible. It is rather surprising, however, that 

 this phenomenon (as I have found) may actually be observed 

 along a definite longitudinal band in the spectrum, about 

 twice the angular width of the distance between the 

 sodium lines and symmetrical with respect to the axis of 

 rotation. It is independent of the width of the slit, provided 

 this is narrow. 



Again let one spectrum be rotated 180° about a given 

 Fraunhofer line {transverse axis), the mean D line for 

 instance. The two coplanar spectra are now mutually re- 

 versed, showing the succession red-violet and violet-red 

 respectively. Interference should take place only along the D 



* Abridged from a forthcoming report to the Carnegie Institution of 

 Washington, D. C. 



