According to Albert Einstein 's prescription from 1905, a light signal is sent at time τ τ 1 from clock 1 to clock 2 and immediately back, e.g. by means of a mirror. Its arrival time back at clock 1 is τ τ 2 . This synchronisation convention sets clock 2 so that the time τ τ 3 of signal reflection is defined to be τ τ 3 = τ τ 1 + 1 2 ( τ τ 2 − − τ τ 1 ) = 1 2 ( τ τ 1 + τ τ 2 ) .