5. Line of vision depends not at all on the motion of the ether, so long as it has a velocity-potential. Hence if this condition is satisfied the theory of aberration is quite simple. _General Statement as to Negative Results in the Subject._ It is noteworthy that almost all the observations which have been made with negative results as to the effect of the Earth's orbital motion on the ether are equally consistent with complete connexion and complete independence between ether and matter. If there is complete connexion, the ether near the earth is relatively stagnant, and negative terrestrial results are natural. If there is complete independence, the ether is either absolutely stationary or has a velocity-potential, and the negative results are, as has been shown, thereby explained. Direct experiment on the subject of etherial viscosity proves that that is either really or approximately zero, and substantiates the "independence" explanation. _Definition of a Ray._ A ray signifies the path of a definite or identical portion of radiation energy--the direction of energy-flux. In other words, it may be considered as the path of a labelled disturbance; for it is some special feature which enables an eye to fix direction: it is that which determines the line of collimation of a telescope. Now in order that a disturbance from A may reach B, it is necessary that adjacent elements of a wave front at A shall arrive at B in the same phase; hence the path by which a disturbance travels must satisfy this condition from point to point. This condition will be satisfied if the time of journey down a ray and down all infinitesimally differing paths is the same. The equation to a ray is therefore contained in the statement that the time taken by light to traverse it is a minimum; or ∫{A,B} ds/V = minimum If the medium, instead of being stationary, is drifting with the velocity _v_, at angle θ to the ray, we must substitute for V the modified velocity V cos ε + _v_ cos θ; and so the function that has to be a minimum, in order to give the path of a ray in a moving medium, is Time of journey = ∫{A,B} ds / V(cos ε + α cos θ) = ∫{A,B} (V cos ε - v cos θ) / (V²(1-α²)) ds = minimum where α is the ratio v/V. _Path of Ray, and Time of Journey, through an Irrotationally Moving Medium._ Writing a velocity-potential φ in the above equation to a ray, that is putting v cos θ = dφ/ds, and ignoring possible variations in the minute correction factor 1-α² between the points A and B, it becomes Time of journey = ∫{A,B} cos ε / (1 - α²) · ds/V - (φβ - φα) / V²( 1-α²) = minimum.