[25] This is because in Michelson’s experiment it is not necessary to consider a sphere. The two arms of the apparatus may be of different lengths; and all that is observed is the continued coincidence of the interference-bands with markings on the instrument.
[26] More precisely
[27] When we consider the
’s in this light, we realise that the expression for
might have been anticipated directly without any regard to measurement. We might, for instance, have attempted to construct in a purely mathematical way the possible expressions containing variables such as
and