tnote 311: _Phil. Trans._, vol. xc., p. 70. With the forty-foot,
however, only very moderate powers seemed to have been employed, whence
Dr. Robinson argued a deficiency of defining power. _Proc. Roy. Irish
Ac._, vol. ii., p. 11.]

[Footnote 312: _Phil. Trans._, vol. lxxi., p. 492.]

[Footnote 313: It is remarkable that, as early as 1695, the possibility
of an achromatic combination was inferred by David Gregory from the
structure of the human eye. See his _Catoptricae et Dioptricae Sphericae
Elementa_, p. 98.]

[Footnote 314: Wolf, _Biographien_, Bd. ii., p. 301.]

[Footnote 315: _Month. Not._, vol. i., p. 153. _note_.]

[Footnote 316: Henrivaux, _Encyclopedie Chimique_, t. v., fasc. 5, p.
363.]

[Footnote 317: See _ante_, p. 83.]

[Footnote 318: _Phil. Trans._, vol. vii., p. 4007.]

[Footnote 319: J. Herschel, _The Telescope_, p. 39.]

[Footnote 320: _Month. Not._, vol. xxix., p. 125.]

[Footnote 321: A slight excess of copper renders the metal easier to
work, but liable to tarnish. Robinson, _Proc. Roy. Irish Ac._, vol. ii.,
p. 4.]

[Footnote 322: _Brit. Ass._, 1843, Dr. Robinson's closing Address.
_Athenaeum_, Sept. 23, p. 866.]

[Footnote 323: _The Telescope_, p. 82.]

[Footnote 324: Lord Rosse in _Phil. Trans._, vol. cxl., p. 302.]

[Footnote 325: This method is the same in principle with that applied by
Grubb in 1834 to a 15-inch speculum for the observatory of Armagh.
_Phil. Trans._, vol. clix., p. 145.]

[Footnote 326: Robinson, _Proc. Roy. Ir. Ac._, vol. iii., p. 120.]

[Footnote 327: _Astr. Nach._, No. 536.]

[Footnote 328: Airy, _Month. Not._, vol. ix., p. 120.]

[Footnote 329: _Astronomical Journal_ (Gould's), vol. ii., p. 97.]

[Footnote 330: _Ibid._, p. 160.]

[Footnote 331: Lord Rosse in _Phil. Trans._, vol. cxl., p. 505.]

[Footnote 332: No. 2343 of Herschel's (1864) Catalogue. Before 1850 a
star was visible in each of the two larger openings by which it is
pierced; since then, one only. Webb, _Celestial Objects_ (4th ed.), p.
409.]

[Footnote 333: _Mem. Am. Ac._, vol. iii., p. 87; _Astr. Nach._, No.
611.]

[Footnote 334: _Pop. Astr._, p. 145.]

[Footnote 335: This statement must be taken in the most general sense.
Supplementary observations of great value are now made at Greenwich with
the altitude and azimuth instrument, which likewise served Piazzi to
determine the places of his stars; while a "prime vertical instrument"
is prominent at Pulkowa.]

[Footnote 336: As ea