ABSTRACT
In the case of accelerators, our demands on the relative motion are mostly that the beam does not become unreasonably large, and hence that the motion is somehow bounded within a suitable volume of phase space. While this appears to be a modest wish for long single pass accelerators, and more so for repetitive systems, this problem actually turns out to be rather nontrivial. This chapter presents a short excursion to a field that at first glance appears disconnected from beam physics, namely the field of glass optics. It shows that glass optics, which has existed long before the name beam physics was introduced, certainly belongs to this field: the ensembles of light particles or rays typically associated with questions of glass optics form a beam not only in the conventional meaning of the word, but also under the more formal definition.
