But on the 16th day of the same month – which happened to be a Monday, and a Council day of the Royal Irish Academy – I was walking in to attend and preside, and your mother was walking with me, along the Royal Canal…An electric circuit seemed to close; and a spark flashed forth, the herald (as I foresaw, immediately) of many long years to come of definitely directed thought and work, by myself if spared, and at all events on the part of others, if I should even be allowed to live long enough distinctly to communicate the discovery. Nor could I resist the impulse – unphilosophical as it may have been – to cut with a knife on a stone of Brougham Bridge, as we passed it, the fundamental formula with the symbols, i, j, k; namely,Letter dated August 5, 1865 from Sir W. R. Hamilton to Rev. Archibald H. Hamilton.
i2 = j2 = k2 = ijk = -1
The classic account of the discover of quaternions. Hamilton also notes in this letter that the Council Books of the Academy record that he had obtained leave to read a paper on quaternions, which reading took place on the 13th November 1843.