REVERSED AND NON-REVERSED SPECTRA. 



23 



Both mirrors, M and AT, were now placed on micrometers moving nearly 

 normal to their faces. Beginning with a coincidence of the illuminated strips 

 on the grating, the M micrometer was moved until the fringes disappeared. 

 The N micrometer was then moved in the same direction, until the reappearing 

 fringes passed through an optimum and finally vanished, in turn. There- 

 after the M micrometer was displaced again, always in the given direction, 

 and the same cycle repeated, etc. It was possible to pass through about 

 8 cycles with each micrometer before the illumination reached the edge of 

 the grating, each cycle corresponding to a displacement of about 2 mm. for 

 a single mirror; but a total displacement of 2.5 cm. was registered, which 

 would obviously have been increased much further if the grating had been 

 wider. The data given in table 8 give a concrete example : 



TABLE 8. 



As both mirrors move in the same direction, the two illuminated strips on 

 the grating gradually separate until they are quite distinct. Meanwhile the 

 fringes pass with rotation from the original sagittate forms to very fine hair- 

 like striations ; whereas the part of the spectrum within which the former 

 occur is less than the distance apart of the sodium lines (doublets) , the hair- 

 lines are visible within a strip of spectrum many times as broad as the sodium 

 doublet. Ten such lines may be visible. In good adjustments the sagittate 

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

 10, .A, arranged or strung on the same major axis. The vertices a are therefore 

 thick and pronounced, but taper rapidly down into hair-lines, 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 exhaustive 

 description of the phenomena; for it is not uncommon, when partial hyper- 

 bolas appear, to find the striations (which are always faint) in the same 

 direction on both sides, as in figure 10, >; i.e., the striations are apt to be 

 non-symmetrical on the two sides, as if they constituted a second diffraction 

 phenomenon superimposed on the first phenomenon. Roof -shaped forms 

 (fig. 10, Q, strongly dotted, are also common, often irregularly awned. 



Figure n may be consulted to further elucidate the subjects under con- 

 sideration. G is the grating, PP' the principal plane of the objective of the 

 telescope, a and b are two rays interfering at the focus /, and leaving the 

 grating parallel and symmetrically placed to the axial ray of. The passage 



