XVI

of the earth in space can be made perceptible in terrestrial experiments. We have already remarked in [chapter:V]Section V that all attempts of this nature led to a negative result. Before the theory of relativity was put forward, it was

difficult to become reconciled to this negative result, for reasons now to be discussed. The inherited prejudices about time and space did not allow any

doubt to arise as to the prime importance of the Galilei transformation for changing over from one

body of reference to another. Now assuming that the Maxwell-Lorentz equations hold for a reference-body $K$,

we then find that they do not hold for a reference-body $K^{\prime }$ moving uniformly with respect to $K$, if we assume that the relations of the Galileian transformation exist between the co-ordinates of $K$ and $K^{\prime }$. It thus appears that of all Galileian co-ordinate systems one ($K$) corresponding to a particular state of motion is physically unique. This result was interpreted physically by regarding $K$ as at rest with respect to a hypothetical æther of space. On the other hand, all co-ordinate systems $K^{\prime }$ moving relatively to $K$ were to be regarded as in motion with respect to the æther.

To this motion of $K^{\prime }$ against the æther ("æther-drift" relative to $K^{\prime }$) were assigned the more complicated laws which were supposed to hold relative to $K^{\prime }$. Strictly speaking, such an æther-drift ought also to be assumed relative to the earth, and for a long time the efforts of physicists were devoted to attempts to detect the existence of an æther-drift at the earth's surface.