Alexandrotype

Source / Recreative Science
Title / Mike Ware

It has been claimed that Alexander the Great exploited light-induced colour changes in a dye, in order to coordinate the battle-order of his troops, which proved crucial to the succcessful outcome of his campaigns. The Macedonian troops supposedly wore bands of rag around their wrists which were impregnated with a photochromic dye. Their exposure to the light of the rising sun caused a colour change which signalled the moment of attack. This device has been referred to as Alexander’s Rag Time Band.

P.S. Yes, this is a joke:-)

Mike Ware is a british chemist and inventor of the New Cyanotype process – amongst others and an author of many books.

Source: Written by Mike Ware and, based upon a piece of mythology that every student of photochemistry learns at an early stage!