ady remarked that since the star passes directly overhead there
should be practically no refraction; and this assumption was made by
Molyneux and Bradley in choosing this particular star for observation. It
follows at once, if we assume that the atmosphere surrounds the earth in
spherical layers. But perhaps this was not so? Perhaps, on the contrary,
the atmosphere was deformed by the motion of the earth, streaming out
behind her like the smoke of a moving engine? No possibility must be
overlooked if the explanation of this puzzling fact was to be got at.

[Illustration: FIG. 3.]

The way in which a deformation of the atmosphere might explain the
phenomenon is best seen by a diagram. First, it must be remarked that rays
of light are only bent by the earth's atmosphere, or "refracted," if they
enter it obliquely.

If the atmosphere were of the same density throughout, like a piece of
glass, then a vertical ray of light, A B (see Fig. 3), entering the
atmosphere at B would suffer no bending or refraction, and a star shining
from the direction A B would be seen truly in that direction from C. But
an oblique ray, D E, would be bent on entering the atmosphere at E along
the path EF, and a star shining along D E would appear from F to be
shining along the dotted line G E F. The atmosphere is not of the same
density throughout, but thins out as we go upwards from the earth; and in
consequence there is no clear-cut surface, B E, and no sudden bending of
the rays as at E: they are gradually bent at an infinite succession of
imaginary surfaces. But it still remains true that there is no bending at
all for vertical rays; and of oblique rays those most oblique are most
bent.

[Illustration: FIG. 4.]

Now, suppose the atmosphere of the earth took up, owing to its revolution
round the sun, an elongated shape like that indicated in diagram 4, and
suppose the star to be at a great distance away to the right of the
diagram. When the earth is in the position labelled "June," the light
would fall vertically on the nose of the atmosphere at A, and there would
be no refraction. Similarly in "December" the light would fall at C on the
stern, also vertically, and there would be no refraction. [The rays from
the distant star in December are to be taken as sensibly parallel to those
received in June, notwithstanding that the earth is on the opposite side
of the sun, as was remarked on p. 98.] But in March and September the rays
would strik