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 of the residual irreversible shift observable at high speeds, when all kinds of mechanical and thermal disturbance have been apparently eliminated.

The question of Electrification.

Although it must new be taken that such messes of matter as we had been dealing with are incompetent to disturb the ether in e rotational manner (for, as has been emphasised in the previous memoir, irrotational motion of a single-valued kind could not be detected by interference or any other optical experiments, since such motion in no way effects either the path or the speed of a ray), and although further it has now been shown that the conveyance of a magnetic field by moving matter confers; no power of gripping the ether, yet it was thought possible that electrification might do it; because an oscillatory charge certainly radiates wave motion into the ether. And although radiation is not any known kind of mechanical disturbance, but is concerned with the ether's electrical properties, end need not necessarily involve anything analogous to etherial viscosity even in the neighbourhood of matter, yet it was thought possible that electric charge, being as it were the connexion between ether and matter, might confer upon a material body some power of gripping and rotationally carrying forward the ether in a quasi-viscous manner. At any rate, whatever reason could be urged for or against such a connexion, it was desirable to bring it to the test of experiment end superpose an electric field upon the moving disks.

The natural plan for electrifying the disks would seem to be to make one of the disks positive and the other negative, but after consideration it was found impracticable to insulate the existing disks satisfactorily; and a third disk half way between the other two was contemplated. This might possibly have been stationery and independently supported, but some preliminary experiments with a plate thus held showed that in the draught of air it developed some warmth and interfered with the



fringes; so a rotating disk, clamped in insulating washers between the other two, was decided on, and made as shown in fig. 7. The new disk wee made an inch smaller in diameter than the others, but otherwise it was just like them. Connexions were arranged through insulating holes for the supply of electricity near the axle