This latest volume of these Advances contains two regular articles and a separate
part in celebration of “50 Years of Electronics.” The two regular contributions
are by authors whose names are known to readers of the series; J.-y. Ximen has
already written an account of his canonical approach to aberration theory, which
he extends here to high order aberrations. Although these very high order perturbations
are not of immediate interest for short systems such as electron microscopes
and similar instruments, they do become important in circular devices
and perhaps in very long structures. I. J. Taneja has likewise described his ideas
about information measures in these pages and, in the present contribution, he
extends and generalizes those notions.