REVERSED AND NON-REVERSED SPECTRA. 



Ill 



60. Jamin's interferometer. The ease with which the Michelson inter- 

 ferometer may be adjusted and its remarkable adaptability have led to its 

 general preference over the older form of Jamin. Nevertheless, the latter 

 furnishes two parallel rays which for such purposes as the present are desir- 

 able. Hence if the four faces of the interferometer be separated in the manner 

 suggested by Mach (shown in fig. 73), a very available form of interferom- 



eter is obtained. Here M and N are half-silvered plates, M' and N' the 

 opaque mirrors. The white light L impinging from a collimator thus fur- 

 nishes the two component beams o<7 and bd, which are observed with the tele- 

 scope at T, after passing the direct-vision prism grating g. If either mirror 

 M' or N' is displaced a distance e, moving parallel to itself, the path-differ- 

 ence ze cos 6 is introduced with the corresponding shift of ellipses. The 

 U-tubes C, C' t with their helices H, H r , and connecting pipe p are now con- 

 veniently installed as shown. But the trouble with the arrangement is the 

 difficulty of adjusting the four surfaces. Not only are the centers of ellipses 

 liable to be remote from the center of the field, but it is often hard, without 

 special equipment, to even find the fringes. 



If, however, the device which I suggested in the preceding report is adopted 

 i.e., if (fig. 74) the half -silvered plates M, N are at the ends of a single 

 strip of plate-glass, so that rays terminating in M, M' N N' after adjust- 

 ment necessarily make a rhombus-like figure symmetrical to MN the fringes 

 are found at once; for they appear when the white slit images in T coincide 

 horizontally and vertically and the rays bd and cd intersect in the common 

 point d. Hence the mirrors M', N' should be on carriages D, F, adapted to 

 move on parallel slides S, S f . M, N may also be put on a carriage E, though 

 this is not necessary. S, S' need not be parallel to ac or bd. If the mirrors 

 M' and N' are wide, considerable latitude of adjustment is thus obtained. 



If MN is half -silvered on the same side (i.e., toward A/ 7 ) a compensator is 

 needed in ac or cd if path-difference is to be annulled (symmetry). If, how- 

 ever, M is half -silvered on the N' side and N on the M' side, no compensator 

 is required. In the latter case, however, if ordinary plate-glass is taken, M 

 and N are not quite parallel and the ellipses will be eccentric. This, however, 

 is not necessarily a disadvantage, unless the strip MN is excessively wedge- 

 shaped. 



