Sect. 13. 203 and the geometric elements for the large orbit are re-computed. From the analysis it cannot be decided whether the third body is close to the visual primary or secondary component; this can only be decided by measuring the orbital motion on an astrometric background of other stars (Chap. IV). In accurate long-term problems the perspective change in the apparent orbit due to motion of the center of mass in the line of sight, must be taken into account. In the future this feature of orbital analysis will undoubtedly become more and more important. An example of the required reduction is the orbital analysis of the 61 Cygni system by A. FLETCHER!. 1890 90° 1960 0° Fig. 11. Relative orbit of the visual binary Kruger 60. * visual observations. 0 photographic observations. IV. Analysis of orbit referred to external reference system. 13. Introduction. In the preceding Chap. III we dealt with the analysis of the relative orbit of the two components of a binary star. We now extend the analysis to the measurements and orbits of the individual components referred to a system of, presumably distant, reference stars. Both micrometer measure­ ments and, particularly, meridian circle positions have been successfully employed in the past; the current observing technique is primarily long-focus photographic astrometry. The problems studied include those of mass-ratio and parallax, and of perturbations. Long-focus technique and analysis are in the first place suitable for the so-called resolved binaries, whose components are clearly separated.