The diagram also equally represents the case of a moving source in a stationary medium. The source, starting at C, has moved to S, emitting waves as it went; which waves, as emitted, spread out as simple spheres from the then position of source as centre. Wave-normal and ray now coincide: S M is not a ray, but only the locus of successive disturbances. A stationary telescope would look not at S, but along M C to a point where the source was when it emitted the wave M; a moving telescope, if moving at same rate as source, will look at S. Hence S M is sometimes called the _apparent_ ray. The angle S M C is the aberration angle, which in Chap. X we denote by ε. Fig. 6 shows normal reflexion for the case of a moving medium. The mirror M reflects light received from S₁, to a point S₂,--just in time to catch the source there if that is moving with the medium. Parenthetically I may say that the time taken on the double journey, S₁ M S₂, when the medium is moving, is not quite the same as the double journey S M S, when all is stationary; and that this is the principle of Michelson's great experiment; which must be referred to later. [Illustration: FIG. 6. Normal Reflexion in Moving Medium. The angle M S X is the angle θ in the theory of Michelson's experiment described in Chapter IV.] The ether stream we speak of is always to be considered merely as one relative to matter. Absolute velocity of matter means velocity through the ether--which is stationary. If there were no such physical standard of rest as the ether--if all motion were relative to matter alone--then the contention of Copernicus and Galileo would have had no real meaning. FOOTNOTE: [3] _Radian_ is the name given by Prof. James Thomson to a unit angle of circular measure, an angle whose arc equals its radius, or about 57°. CHAPTER IV EXPERIMENTS ON THE ETHER We have arrived at this: that a uniform ether stream all through space causes no aberration, no error in fixing direction. It blows the waves along, but it does not disturb the line of vision. Stellar aberration exists, but it depends on motion of observer, and on motion of observer only. Etherial motion has no effect upon it; and when the observer is stationary with respect to object, as he is when using a terrestrial telescope, there is no aberration at all. Surveying operations are not rendered the least inaccurate by the existence of a universal etherial drift; and they therefore afford no means of detecting it.