The concept of geomagnetic optics, as described by the asymptotic directions of approach, is extremely useful in the analysis of cosmic radiation data. However, when changes in cutoff rigidities occur as a result of evolution in the geomagnetic field, there are corresponding changes in the asymptotic cones of acceptance. It has been demonstrated that the effect of geomagnetic disturbances on asymptotic directions of approach can be estimated for nonpolar locations from the corresponding change in the cutoff rigidity. In this paper we show that the same procedure is applicable with respect to the secular evolution in the geomagnetic field, and that with this procedure it is possible to adjust reference published asymptotic directions to other epochs if the change in cutoff rigidity is known. ¿American Geophysical Union 1987