[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° 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.