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is $T'$, and the perpendicular to the incident waves has the direction constants $b_{x},b_{y},b_{z}$ that are to be determined by (71).

Thus all phenomena happen exactly in such a manner, as if the earth were at rest, the oscillation period ware T', and the celestial body, as seen from earth, would be located not in the direction ($-b'_{x},-b'_{y},-b'_{z}$), but in the direction ($-b_{x},-b_{y},-b_{z}$).

Now, aberration exactly consists of the latter. That the magnitude and direction, which we find for it, actually corresponds to the known rule which is in accordance with observations, follows immediately from equation (71). Namely, we obtain a vector of direction ($b_{x},b_{y},b_{z}$), when we compose a vector of direction ($b'_{x},b'_{y},b'_{z}$), whose length represents the velocity of light, with a second one which is equal and opposite to Earth's velocity ${\mathfrak {p}}$.

By the way, in our theorem also lies the explanation for the fact, that during the observation by a lens system, always that aberration arises which is determined by the previously mentioned rule^[1], as well as the explanation for the known experiments of Arago^[2] by a prism, and for the experiment proposed by Boscovich and executed by Airy, in which the tube of a telescope was filled with water^[3].

Observations by sun light.

§ 62. The trajectory of Earth deviates as little from a circle,

1. That this is also the case during the observation by a mirror telescope, would also follow from our theorem, when the mirror would consist of transparent material. However, as regards the actual mirrors that are constructed by metal, we can remark, that the direction by which the light rays are reflected, and the location of the unification point only depends on the curvature, but not on the material nature of the mirror. For the determination of this location, as it was done by various physicists, also the Principle of Huygens can be applied, (see also my treatise in Arch. neerl., T. 21).
2. Arago. OEuvres completes, T. 1, p. 107; Biot. Traité élémentaire d'astronomie physique, 3e éd., T. 5, p. 364.
3. Airy. Proc. Royal Society of London, Vol. 20, p. 35, 1871; Vol. 21, p. 121, 1873; Phil. Mag., 4th Ser., Vol. 43, p. 310, 1872.