Don't think I saw this thread first time round. Many years ago, in the early days of home computing (think Apple II, Acorn Atom, BBC Micro etc.), long before the Internet, an optician friend of mine asked me to produce a table of values for some purpose to do with producing spectacle lenses. There was a simple formula involved - so away I went, no problem. As a check, I dug out an old book of such info for opticians - and found that my table matched everywhere - except in one obscure corner of the data! "That's alright" said my friend - that combination of values would never be used in practice." I subsequently discovered that publishers of such tables often included such errors deliberately - as a marker of their own versions in case they were copied! Apparently done in other, similar areas also.


Regards