CHAPTER I. 



THE INTERFERENCES OF CROSSED SPECTRA. 



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 two component rays are not so great as to throw the phenomenon 

 beyond the range of visibility. In the usual method of producing these 

 interferences, where the corresponding 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 

 separated in some other way; and the problem of centering is often one of 

 the chief difficulties involved; and if the 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, how far 

 can one trench with light-waves upon the case of musical beats, or of inter- 

 ferences 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 around a longitudinal axis, 

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

 hofer lines. In such a case, interference should be possible only along the 

 infinitely thin longitudinal axis of rotation to which both spectra are sym- 

 metrical, 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 

 enough to show the Fraunhofer lines to best advantage. 



Again, let one spectrum be rotated 180 about a given Fraunhofer line 

 (transverse axis), the nickel or mean D line, for instance. The two coplanar 

 spectra are now mutually reversed, showing the succession red- violet and 

 violet-red, respectively. Interference should take place only along the mean 

 D line and be again inappreciable. Experimentally, I was not at first able 

 to find any interferences for this case in the manner shown below, but this 

 may have been due to inadequacies in the experimental means employed, 

 for the dispersion was insufficient and the reflecting edge of the paired mirrors 

 too rough. Improving the apparatus, I eventually found the phenomenon, 



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