or, to the first order of minutiæ, (μ - 1)z - αz cos θ; θ being the angle between ray and ether drift inside the medium. So the extra equivalent air layer _due to the motion_ is approximately ±α z cos θ, a quantity independent of μ. Hence, no plan for detecting this first-order effect of motion is in any way assisted by the use of dense stationary substances; their extra ether, being stationary, does not affect the lag caused by motion, except indeed in the second order of small quantities, as shown above. Direct experiments made by Hoek,[11] and by Mascart, on the effect of introducing tubes of water into the path of half-beams of light, are in entire accord with this negative conclusion. Thus, then, we find that no general motion of the entire medium can be detected by changes in direction, or in frequency, or in phase; for on none of them has it any appreciable (i.e. first-order) effect, even when assisted by dense matter. * * * * * Another mode of stating the matter is to say that the behaviour of ether inside matter is such as to enable a potential-function, ∫ μ²v cosθ ds, to exist throughout all transparent space, so far as motion of ether alone is concerned. (See Appendix 3.) The existence of this potential function readily accounts for the absence of all effect on direction due to the general drift of the medium, whether in the presence of dense matter (such as water-filled telescopes) or otherwise. Whatever may be the path of a ray by reason of reflexion or refraction in a stationary ether, it is precisely the same in a moving one if this condition is satisfied, although the wave-normals and wave-fronts are definitely shifted. However matter affects or loads the ether inside it, it cannot on this theory be said either to hold it still, or to carry it with it. The general ether stream must remain unaffected, not only near, but inside matter, if rays are to retain precisely the same course as if it were relatively stationary. But it must be understood that the etherial motion here contemplated is the _general drift of the entire medium_; or its correlative, the uniform motion of all the matter concerned. There is nothing to be said against aberration effects being producible or modifiable by motion of _parts_ of the medium, or by the artificial motion of transparent bodies and other partitioned-off regions. _Artificial_ motion of matter may readily alter both the time of journey and the path of a ray, for it has no potential conditions to satisfy; it may easily describe a closed contour, and may take part in conveying light.