CHAPTER II. 



THE INTERFERENCES OF INVERTED SPECTRA. 



28. Introductory. If two identical spectra are superposed in such a way 

 that one is rotated 180, on a transverse axis (parallel to the Fraunhofer 

 lines), with respect to the other, it will be convenient to refer to the phenomena 

 resulting and described in Chapter I as the interferences of reversed spectra. 

 On the other hand, if one of the superposed spectra has been rotated 180 

 with respect to the other on a longitudinal axis (parallel to the length of the 

 spectrum), we may refer to the interferences as those of inverted spectra. 

 The absence of either of these adjustments would then be accentuated as 

 non-reversed or non-inverted. 



In the case of inverted spectra, therefore, we are dealing with phenomena 

 of virtually homogeneous light, exhibited throughout the length of the spec- 

 trum. Such experiments were made cursorily in my first paper on the sub- 

 ject,* but the phenomenon is very peculiar, apparently anomalous, and further 

 treatment is therefore desirable. 



29. Apparatus. Non=inverted spectra. The apparatus formerly used for 

 long optical paths is difficult to manipulate. It has therefore been simplified 

 in the present method and used for short distances, so as to be wholly in the 

 observer's control. The parts of the apparatus are conveniently assembled as 

 in a preceding experiment, except that the slit which furnishes the collimated 

 light L, figure 42, is now horizontal i.e., at right angles to the edge of the 

 sharp prism P (about 20 at apex). The rays a and a' (horizontal blades), 

 after leaving the opaque mirrors M and N, are then reflected from the sides 

 of the right-angled prism P' into c and c' . Thus far the light is white; but 

 c and c' are now diffracted by the grating G with its rulings horizontal (par- 

 allel to slit) and with the interposition of an auxiliary prism p (edge hori- 

 zontal), the two spectra due to c and c' are observed by a telescope at T. 



The experiment succeeds best with sunlight. The triangle of rays, a, a', 

 b', &,is first made isosceles and horizontal by adjusting P,P', being set midway 

 and symmetrically between M and N. The two spectra in the telescope 

 (vertical ribands) are now made to overlap at their inner edges and the two 

 sodium doublets placed accurately in contact, by aid of the adjustment 

 screws on the mirrors M and N. The latter are then to be moved micromet- 

 rically (Fraunhofer slide) until the fringes appear. The experiment is not 

 an easy one. 



It is obvious from figure 42 that the two superimposed spectra are non- 



* Am. Journal, XL, pp. 486 to 498, 1915, 4; Carnegie Inst. Wash. Pub. 249, Chap. I. 

 62 



