PREFACE. 



In the present volume I have pursued the work on the interferences of 

 reversed and non-reversed spectra, begun in my last report (Carnegie Inst. 

 Wash. Pub. No. 249, 1916), in a variety of promising directions, such as the 

 original investigation suggested. It will be remembered that the reversal 

 (180) here contemplated takes place on a transverse line of the spectrum 

 (i.e., a line parallel to the Fraunhofer lines), which thereby becomes a line 

 of symmetry for the phenomena. The apparatus has been extensively modi- 

 fied, so as to admit of measurements relating to individual fringes. The 

 object of such quantitative work, however, is to furnish a guide for the devel- 

 opment of the experiments and to corroborate equations, not to collate 

 standard data. These could hardly be satisfactorily obtained, moreover, 

 unless the work were done with optical plates and mirrors, whereas the work 

 in this volume and the preceding has been done with ordinary window-plate 

 and usually with film gratings. 



A large part of Chapter I is devoted to the treatment of prismatic methods, 

 developed with the additional purpose of securing a greater intensity of light. 

 A very curious intermediate case between the interferences of reversed and 

 non-reversed spectra is the pronounced interference of spectra from the same 

 source, but of different lengths (dispersion) between red and violet. The 

 phenomena of crossed rays find a parallel occurrence in the present paper, 

 in the behavior of duplicated fringes, when similar gratings or prisms disperse 

 and subsequently recombine a beam of white light. A type of fringes is 

 detected which depends merely on the grating space and is independent of 

 wave-length. An interesting question as to the limits of micrometer displace- 

 ment within which fringes of any kind are discernible (observations which 

 were at first supposed to be due to the degree of uniformity of interfering 

 wave-trains) is eventually shown to be a necessary result of dispersion. 

 Finally, the direct interference of divergent rays obtained from polarizing 

 media is exhibited. 



In Chapter II the interferences of inverted spectra, a subject merely 

 touched in the preceding volume, are given greater prominence. In this 

 case one of the two spectra from the same source is inverted (180) relatively 

 to the other on a longitudinal axis (i.e., an axis normal to the Fraunhofer 

 lines), which thus becomes a line of symmetry. In the development of the 

 subject, spectra half reversed and spectra both reversed and inverted are 

 treated successfully. In the latter case the conditions of interference are 

 fulfilled at but a single point in the whole area of the spectrum field; and 

 yet the phenomenon is pronounced and not very difficult to realize. The 

 limits of micrometer displacement within which interferences may be obtained 

 are again determined. At the end of the chapter it was thought useful to 

 collate available equations in the treatment of phenomena of the present kind. 



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