[308] Ibid., vol. xc., p. 65.
[309] Cassegrain, a Frenchman, substituted 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.
[310] Phil. Trans., vol. civ., p. 275, note.
[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.
[312] Phil. Trans., vol. lxxi., p. 492.
[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 Catoptricæ et Dioptricæ Sphericæ Elementa, p. 98.
[314] Wolf, Biographien, Bd. ii., p. 301.
[315] Month. Not., vol. i., p. 153. note.
[316] Henrivaux, Encyclopédie Chimique, t. v., fasc. 5, p. 363.
[317] See ante, p. 83.