The theory of Professor H.A. Lorentz, accordingly, shows that the shape of Michelson's stone supporting block will be distorted by the motion; its dimensions across and along the line of ether drift being affected differently. And the amount of the change will be such as precisely to compensate and neutralise the optical effect of motion which might otherwise be perceived. This theory is now generally accepted. It is this neutralising or compensatory effect,--which acts equally on to-and-fro motion of light, to-and-fro motion of electric currents, and on the shape of material bodies,--that renders any positive result in experiments on ether-drift so difficult or impossible to obtain; so that, in spite of the speed with which we are rushing through space, no perceptible influence is felt on either electrical or optical phenomena, except those which are due to relative motion of source and observer. _Some Details in the Theory of the Doppler Effect, or Effect of Motion on Dispersion by Prism or Grating._ When light is analysed by a prism or grating into a spectrum, the course of each ray is deflected--refracted or diffracted--by an amount corresponding to its frequency of vibration or wave-length. Motion of the medium, so long as it is steady, affects neither frequency nor wave-length, and accordingly is without influence on the result. It produces no Doppler effect except when waxing or waning. Motion of the source alone crowds the waves together on the advancing side and spreads them out on the receding side. An observer therefore whom the source is approaching receives shorter waves, and one from whom the source is receding receives longer waves, than normal. At any fixed point waves will arrive, therefore, with modified frequency. So long as a source is stationary the wave-lengths emitted are quite normal, but motion of an observer may change the frequency with which they are _received_, in an obvious way; they are swept up faster if the receiver is approaching, they have a stern chase if it is receding. All this is familiar, and was geometrically illustrated in Chapter III, but there are some minor and rather curious details which are worthy of brief consideration. _Grating Theory._ For suppose a 'grating' is used to analyse the light. Its effect can depend on nothing kinetic; it must be regulated by the merely geometric width of the ruled spaces on it. Consequently it can only directly apprehend wave-lengths, not frequencies. In the case of a moving _source_, therefore, when the wave-length is really changed, a grating will appreciate the fact, and will show a true Doppler effect. But in the case of a moving _observer_, when all the waves received are of normal length, though swept up with abnormal frequency, the grating must still indicate wave-length alone, and accordingly will show no true Doppler effect.