Excerpt from The Interferometry of Reversed and Non-Reversed Spectra, Vol. 2

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 relatively to the other on a longitudinal axis 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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