The multiplication of complex numbers encodes the geometric meaning of rotation in the plane. William Rowan Hamilton attempted to extend this result to the three-dimensional space by considering triplets of numbers at first. He eventually realized that the idea had to be taken further by considering quadruples instead: it was the birth of the quaternions. The plate says:

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

quaternion multiplication

i^2 = j^2 = k^2 = i j k = − 1

& cut it on a stone of this bridge.