REVERSED AND NON-REVERSED SPECTRA. 



121 



central set could be seen. The white field between the patterns was several 

 times the width of the fringed field. Each group as a whole resembles the 

 fringes of the biprism, as usual, and they differ appreciably only in intensity 

 and in their focal planes. It is difficult to account for this periodic reappear- 

 ance; but it must be due to reflections at the half -silver surfaces. The reflec- 

 tions from the uncovered surfaces are indeed just visible, and naturally, since 

 they are duplicates, they also carry the fringes. But they are easily differ- 

 entiated by the relative faintness of field and have nothing to do with the 

 recurrences in question. 



Ordinary daylight is quite adequate to show the residual fringes in the 

 complete absence of the collimator. They are superimposed on the field of 

 view (landscape, etc.) and hence will subserve other purposes than are here 

 given. When the adjustment for parallelism is not sharp, the fringes may 

 often be found strong in continuously varying focal planes. 



64. Wide slit. Homogeneous light. Sodium flame. A further clue to 

 the nature of the residual fringes will be obtained when white light is replaced 

 by homogeneous light. A strong, large sodium flame near the mirror M, 

 figure 73, suffices. The fringes now appear of the same 

 size in yellow light, naturally spread over a much 

 larger area of field. But on moving the mirror M' 

 (AJV increasing continually) forward very gradually, 

 the homogeneous fringes alternately vanish and reap- 

 pear, each time, however, enlarged in size (nearly 

 doubled but still straight) until at an intermediate 

 position of symmetry enormous round ovals cover the 

 yellow field. The fringes then diminish symmetrically 

 in the same way. The following data for the microm- 

 eter position corresponding to ithe clearest demarca- 

 tions of fringes are illustrative. At least six periods 

 (w) are easily detected on each side of the ovals 

 (n = o) . Thus (originally small fringes, vertical, increasing in size to huge ovals) 



n =6 $ 4 3 2 i 



AJVXio 3 = o 49 90 127 171 214 



o, etc. 

 243 cm., etc. 



These intervals, since it is impossible to establish the maximum states of 

 presence or absence of fringes quite sharply, are practically equidistant, as 

 figure 80 indicates. Thus the mean period of reappearance is AA/" = 0.042 cm.; 

 or a path-difference of 2A7Vcos 6 = 0.059 cm.; or a shift of ray parallel to 

 itself (2A/Vsin 6 = 0.059 cm.) of the same amount. 



The reason for this rhythm can only be the two wave-lengths of the A 

 and Di lines of the sodium flame, originally detected in the colors of thin plates 

 by Fizeau. Hence a relatively enormous shift of micrometer of nearly 0.5 mm. 

 is equivalent to the wave-length interval AX = 6XiQ- 8 cm., or AX/AA/" = 



