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opposite amount for the reverse direction; or, if we set for a match in the one system, the other system would show a change in phase of $0.3\times 10^{-4}\lambda \div 2$, which is seven times the required effect $10^{-4}$, the aberration constant. With condensers larger than needed for the optical condition, 50,000,000 oscillations have been obtained. Thus

$d={\frac {1}{2}}L={\frac {3\times 10^{8}M}{2\times 0.5\times 10^{8}}}=3M$,

a distance small enough to allow a rotating mount for the system. Higher frequencies are undoubtedly possible; so that, if the above optical sensibility is not attainable, we can still use a rotating support, with a reduction in this factor of, say, ten times. This method thus contains the requisite conditions, if all the experimental difficulties in connexion with the uniformity of the oscillation and the source of light can be overcome. The present stage of the work seems to warrant this conclusion.

Physical Laboratory,

University of Nebraska, Lincoln.

April 26, 1905.