Non-reversed Spectrum Interferometry. 



417 



As both mirrors move in the same direction, the two illu- 

 minated strips on the grating gradually separate, until they are 

 quite distinct. Meanwhile the fringes pass from the original 

 sagittate forms to very fine hairlike striations, with rotation. 

 Whereas the part of the spectrum within which the former 

 occur is less than the distance apart of the sodium lines 

 (doublets), the hairlines are visible within a strip of spectrum 

 many times as broad as the sodium doublet. Ten such lines 



Figs. 10, 11, 12, 13. 



i% 



may be visible. In good adjustments the sagittate forms are 

 seen to be a nest of very eccentric, identical hyperbolas, as in 

 fig. 10, arranged or strung on the same major axis. The ver- 

 tices, a, are therefore thick and pronounced, but taper rapidly 

 down into hairlines, b, b', on both sides. Frequently but half 

 of the coarse vertices, a, abundantly fringed on one side, b or 

 b', appear. Nevertheless this does not seem to be an exhaus- 

 tive description of the phenomena, for it is not uncommon, 

 when partial hyperbolas appear, to find the striations (which 

 are always faint) in the same direction on both sides, as in 

 fig. 11 ; i. e. the striations are apt to be nonsymmetrical on 

 the two sides, as if they constituted a second diffraction 



