On the 16th October 1843 the Irish mathematician William Hamilton was taking a walk with his wife, alongside the Royal Canal in Dublin, when the answer to a problem that he had been puzzling over came to him, and he was so excited by this discovery that he carved the equation:
I2 = j2 = k2 = ijk = -1
on a stone of the nearest bridge.
History does not record the reaction of his wife to this act, but judging by his recollection of his conversation with his sons:
Every morning in the early part of the above-cited month, on my coming down to breakfast, your (then) little brother William Edwin, and yourself, used to ask me: “Well, Papa, can you multiply triplets?” Whereto I was always obliged to reply, with a sad shake of the head: `No, I can only add and subtract them”.
I suspect that she was not unduly surprised.
The graffiti was soon worn away, but the event was later recorded more permenantly with a commemerative stone with the words:
Here as he walked by
on the 16th of October 1843
Sir William Rowan Hamilton
in a flash of genius discovered
the fundamental formula for
i2 = j2 = k2 = ijk = -1
& cut it on a stone of this bridge
For more information look here: http://math.ucr.edu/home/baez/dublin/ for an account of a trip to the bridge by mathematician John Baez and friends, including some nice photographs of the structure itself.
Another site with more on the mathematical significance of the quaternion, and a quote from Hamilton himself: http://adaptivecomplexity.blogspot.com/2007/06/science-in-against-day-vectors-and.html