Our system of coordinates is a \emph{static system}, that is to say the $g_{\mu\nu}$ do not change with the time. (An arbitrary coordinate-system has not generally this property; and further when we have to take account of two or more attracting bodies, it is in most cases impossible to find a strictly static system of coordinates.) Taking an observer at rest in the system $(r, \theta, \phi, t)$ a wave emitted by one of the atoms will reach him at a certain time~$\delta t$ after it leaves the atom; and owing to the static condition this time-lag remains constant for subsequent waves. Consequently the waves are received at the same time-periods as they are emitted. We are therefore able to compare the time-periods $dt$ of the different atoms, by comparing the periods of the waves received from them, and can verify experimentally their dependence on the value of~$\sqrt{\gamma}$ at the place where they were emitted. Naturally the most hopeful test is the comparison of the waves received from a solar and a terrestrial atom whose periods should be in the ratio $1.00000212 : 1$. For wave-length $4000$~Å, this amounts to a relative displacement of $0.0082$~Å of the respective spectral lines. The verdict of experiment is not yet such as to secure universal assent; but it is now distinctly more favourable to Einstein's theory than when \Title{Space, Time and Gravitation} was written. The quantity~$dt$ is merely an auxiliary quantity introduced through the equation~\Eq{(38.8)} which defines it. The fact that it is carried to us unchanged by light-waves is not of any physical interest, since $dt$~was \emph{defined} in such a way that this must happen. The absolute quantity~$ds$, the interval of the vibration, is not carried to us unchanged, but becomes gradually modified as the waves take their course through the non-Euclidean space-time. It is in transmission through the solar system that the absolute difference is introduced into the waves, which the experiment hopes to detect.