Summary.
21. Collecting these statements together, we may summarize them thus:—
Source alone moving produces—
A real and apparent change of colour;
A real but not apparent error in direction;
No lag of phase, except that appropriate to altered wave-length;
A change of intensity corresponding to different wave-length.
Medium alone moving, or
Source and receiver moving together
No change of colour;
No change of direction;
A real lag of phase, but undetectable without control over the medium;
A change of intensity corresponding to different virtual distance, but probably compensated by change of radiating power.
Receiver alone moving gives—
An apparent change of colour;
An apparent change of direction;
No change of phase, except that appropriate to extra virtual speed of light;
A change of intensity corresponding to different virtual velocity of light.
Thus the interference effect and the Doppler effect do not occur together. Motion of the medium produces one; motion of source or of receiver produces the other.
Aberration of direction and of pitch occur simultaneously, but are complementary to each other, since one depends on motion across the line of sight, the other on motion along it. One varies as the sine, the other as the cosine, of the inclination. Further discussion of the Doppler effect is deferred to §§ 53–58.
22. It is noteworthy that not one of the methods is able to establish the existence or non-existence of a general ethereal drift near the earth; for, as shown above, uniform motion of the entire medium produces no observable first-order effect of any kind. It plainly becomes the more necessary to attend minutely to possible second- order effects.
In a paper in the ‘Archives Néerlandaises,’ vol. 21, Professor H. A. Lorentz discusses, with much power, the whole subject of ether movement; the idea of the following method of treatment is derived from that paper.
