Lewis & Short

spĕcŭlāris, e, adj. [speculum].

  1. I. In gen., of or belonging to a mirror, like a mirror (post-Aug.): speculari ratione, in the manner of a mirror, Sen. Q. N. 1, 5, 9.
  2. II. In partic.: specularis lapis, a kind of transparent stone, muscovy-glass, isinglass-stone, mica, Plin. 36, 22, 45, § 150 sqq.; 9, 35, 56, § 113; Petr. 68; Lact. Opif. Dei, 8 med.
    Hence, spĕcŭlārĭa, ōrum, n., window panes, a window, Sen. Ep. 90, 25; 86, 11; id. Prov. 4, 9; id. Q. N. 4, 13, 7; Plin. 19, 5, 23, § 64; Plin. Ep. 2, 17, 4 and 21; Mart. 8, 14; Juv. 4, 21; Col. 11, 3, 52; Pall. 1, 20, 1 al.
    In sing.: per corneum specular, Tert. Anim. 53 med.