Part 14 (2/2)

[Footnote 305: Grant, _Hist. Astr._, p. 527.]

[Footnote 306: _Optica Promota_, p. 93.]

[Footnote 307: _Phil. Trans._, vol. x.x.xii., p. 383.]

[Footnote 308: _Ibid._, vol. xc., p. 65.]

[Footnote 309: Ca.s.segrain, a Frenchman, subst.i.tuted in 1672 a _convex_ for a _concave_ secondary speculum. The tube was thereby enabled to be shortened by twice the focal length of the mirror in question. The great Melbourne reflector (four feet aperture, by Grubb) is constructed upon this plan.]

[Footnote 310: _Phil. Trans._, vol. civ., p. 275, _note_.]

[Footnote 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. a.s.s._, 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.]

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