REVERSED AND NON-REVERSED SPECTRA. 



57 



If the fringes are to be used for practical purposes, great care must be taken 

 to keep the angle of incidence of the impinging light constant. This was not 

 done in the present paper, where the purpose is merely an identification of the 

 phenomena. Moreover, a micrometer with the screw running easily is essen- 

 tial, as otherwise the frame is liable to show appreciable twist (change of inci- 

 dence) during displacement of the fringes. 



The fringes are not of the sensitive type, but they admit of a large range of 

 displacement and are therefore adapted to special purposes. 



With regard to their bearing on the behavior of reversed spectra, for the 

 interpretation of which the present experiments were undertaken, it is obvious 

 that the interferences with homogeneous light and a wide slit (paragraph 21), 

 or in the absence of a slit (paragraph 23), are of analogous origin in both 

 cases. It makes no difference, therefore, whether one of the spectra is reversed 

 or not, except, perhaps, that in the former case (inversion), the coincidence 

 of longitudinal and transverse axes is a more insistent condition. The colored 

 fringes of paragraph 20 obviously can not be produced with reversed spectra. 

 There remain the fringes with the fine slit and homogeneous light (paragraph 

 22); in other words, the occurrence of a sort of generalized Fresnellian inter- 

 ferences, within the telescope, modified by causes which lie outside of it. Thus 

 DI and D'i or D 2 and D\ may be placed sufficiently close together to pro- 

 duce a region of interference before and behind the principal plane in which 

 the sodium lines are in focus. If the DiD'i lines are o.oi cm. apart and the 

 fringes seen likewise at o.oi cm. apart, their position, measured from the 

 principal plane, will be at 



YC 



icr 4 5 

 : 5=-cm. 

 6Xicr 3 



or less than 2 cm. The ocular would then have to be displaced forward or 

 rearward by this amount. But there are two sodium doublets, each pair of 

 which is to interfere. Suppose that D 2 and D'\ are in coincidence so that the 



40 



scheme is Di:D*D\: D' 2( as in figure 39, where o is the principal plane of the 

 objective and DI to D' 2 the principal focal plane. We should then have the 

 separate regions of interference I and I' and the combined regions I" and ]'" . 

 When the breadth of the latter is the whole number of fringes, the two pat- 

 terns clearly merge into a single pattern. The experiments show several of 

 these stages, terminating outermost in the focal plane of the colored fringes 

 under the given conditions. Since the fringes lie on hyperbolic loci the problem 



