REVERSED AND NON-REVERSED SPECTRA. 



53 



fringes may be made to vanish under inverse conditions. Table 4 shows their 

 close relation to the preceding colored set, so far as motion of the micrometer 



is concerned. 



TABLE 4. 



Ives grating. 15,050 lines to inch, computed 5e=io- 3 Xo.94 cm. per fringe. 

 io 3 5e=o.72 cm. (large fringes) io 3 6e=o.83 cm. (small fringes) 



.80 

 77 



.80 



Wallace grating. Wide slit. Coincident Na lines. Fringes in principal focus, 



very clear and strong. 

 io 3 de=i.i6 cm. io 3 <5e=i.o6 cm. 



,, Ti.i6 1.08 



Mn g es |; : ; 7 ' 



Wallace grating. Wide slit. Non-coincident Na lines. 



io 3 5e=i.34 cm. io 3 de=i.^8 cm. 



(small fringes) 1.28 (large fringes) 1. 35 



The fringes decrease in size as e increases and exhibit the same irregularity 

 of Se values, due, no doubt, to the same causes (equation 6). Moreover, be 

 is here below the normally computed value, supposing the angle i to be negli- 

 gible. In fact, figure 36 shows the optical center of the collimator C; so that 



37 



36 



Ca and Ca' are the axes of parallel pencils, diffracted by the gratings G and G' 

 at the angles 6 for Ca and tf for Ca' . The rays are subsequently condensed 

 at F, the focus of the telescope, L being the principal plane of the objective. 

 The general path-difference is thus, by equation (5), e(i cos (0'+*))/cos i, 

 which distributes the fringes from right to left with variation of i. 



