  The micro-arcsecond scale structure of the seemingly point-like images in
lensed quasars, though unobservable, is nevertheless much studied
theoretically, because it affects the observable (or macro) brightness, and
through that provides clues to substructure in both source and lens. A curious
feature is that, while an observable macro-image is made up of a very large
number of micro-images, the macro flux is dominated by a few micro-images.
Micro minima play a key role, and the well-known broad distribution of macro
magnification can be decomposed into narrower distributions with 0,1,2,3,...
micro minima. This paper shows how the dominant micro-images exist alongside
the others, using the ideas of Fermat's principle and arrival-time surfaces,
alongside simulations.
