efracting the light from the star. But, so far, our
observations do not indicate this with certainty.

[10] I owe my knowledge of this subject to Dr. G. Johnstone Stoney,
F.R.S. There has been some controversy as to who originated the
ingenious and instructive doctrine here sketched.

[11] The space described by a falling body is proportional to the
product of the force and the square of the time. The force varies
inversely as the square of the distance from the earth, so that the
space will vary as the square of the time, and inversely as the square
of the distance. If, therefore, the distance be increased sixty-fold,
the time must also be increased sixty-fold, if the space fallen through
is to remain the same.

[12] See Newcomb's "Popular Astronomy," p. 78.

[13] Recent investigation by Newcomb on the motion of Mercury have led
to the result that the hypothesis of a planet or a ring of very small
planets between the orbit of Mercury and the sun cannot account for the
difference between theory and observation in the movements of Mercury.
Harzer has come to the same result, and has shown that the disturbing
element may possibly be the material of the Solar Corona.

[14] "The Sun: its Planets, and their Satellites." London: 1882 (page
147).

[15] James Gregory, in a book on optics written in 1667, had already
suggested the use of the transit of Venus for this purpose.

[16] _See_ "Astronomy and Astrophysics," No. 128.

[17] _See_ "Astronomy and Astrophysics," No. 128.

[18] This is the curved marking which on Plate XVIII. appears in
longitude 290 deg. and north of (that is, below) the equator. Here, as in
all astronomical drawings, north is at the foot and south at the top.
_See_ above, p. 82 (Chapter III.).

[19] Now Director of the Lick Observatory.

[20] The heliometer is a telescope with its object-glass cut in half
along a diameter. One or both of these halves is movable transversely by
a screw. Each half gives a complete image of the object. The measures
are effected by observing how many turns of the screw convey the image
of the star formed by one half of the object-glass to coincide with the
image of the planet formed by the other.

[21] See "Astronomy and Astrophysics," No. 109.

[22] It is only right to add that some observers believe that, in
exceptional circumstances, points of Jupiter have shown some slight
degree of intrinsic light.

[23] Professor Pickering, of Cambridge, Mass., has, howe