On Thu, 31 Aug 2000, Michael Koch wrote: > Hi Dominic-Luc, > > Do not try to derive it using the angles between the light and the mirror > surface. This will be difficult. > > Michael Well, actually, that is basically what I tried and realized was difficult, yes, and in fact could not arrive at that equation. Cheers, Dominic North 59 37' 30" East 17 48' 10"