PREFACE. 



The following account of my experiments has been given chronologically. 

 Although many of the anomalous features, in which the interferences of 

 superposed coordinated spectra first presented themselves, were largely 

 removed in the later work, yet the methods used in the several papers, early 

 and later, are throughout different. It therefore seemed justifiable to record 

 them, together with the inferences they at first suggested. The pursuit of 

 the subject as a whole was made both easier and more difficult by the un- 

 avoidable tremors of the laboratory in which I am working; for it is possibly 

 easier to detect an elusive phenomenon if it is in motion among other similar 

 stationary phenomena. But it is certainly difficult, thereafter, to describe 

 it when found. 



It will be convenient to refer to the cases in which one of the two coincident 

 spectra from the same source is rotated 180 with reference to the other on 

 a transverse axis (i.e., an axis parallel to the Fraunhofer lines), under the 

 term reversed spectra; while the term inverted spectra is at hand for those cases 

 in which one of the paired spectra is turned 180 relative to the other on a 

 longitudinal axis (i.e., an axis parallel to the r-v length of the spectrum). 

 In this book the latter are merely touched upon, briefly, in Chapter I, but 

 they are now being investigated in detail and give promise of many interest- 

 ing results. The chapter contains a full account of what may be seen with 

 a single grating the linear phenomenon, as I have called it, and which, if 

 it stood alone, would be difficult to interpret. 



In Chapter II, therefore, the interferences of reversed spectra are treated 

 by the aid of two gratings, in virtue of which a multitude of variations are 

 inevitably introduced. The phenomena are thus exhibited in a way leading 

 much more smoothly to their identification. 



This endeavor is given greater promise in Chapter III, which contains a 

 comparison of the interferences of reversed and non-reversed spectra, the 

 latter produced in a way quite different from those in my earlier work. Nat- 

 urally these in their entirety are even more bewilderingly varied, and become 

 particularly so when, as in Chapter IV, an intermediate reflection of one 

 spectrum is admitted. But with this I was on more familiar ground, as I 

 have hitherto, in these publications, given such investigations particular 

 attention. 



The flexibility of the new methods is well shown in Chapter V, where 

 separated component beams can with equal facility be made to run in parallel, 

 or across each other at any angle, and perhaps both, with the double result 

 visible in the field of the telescope. In case of crossed rays a remarkable 

 phenomenon is shown, in which very small differences in wave-length imply 

 a remarkably large difference in rotational phase (virtually resolving power) 

 of the two interesting groups of interference fringes due to each wave-length. 



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