Umbral

a. [f. UMBRA¹ + -AL.]

  1.  Algebra. Based on the use of umbræ in notation; consisting of umbræ.

1851.  Sylvester, in Lond., etc. Phil. Mag., April, 297. My system of umbral or biliteral notation. Ibid. (1852), in Cambr. & Dubl. Math. Jrnl., VII. 78, note. The umbral method of denoting such a function.

1893.  Cayley, Math. Papers (1897), XIII. 306. It does not appear that there is any monomial umbral expression for the last-mentioned form.

  2.  Astr. Pertaining to the umbra of sun-spots or eclipses.

1867–77.  G. F. Chambers, Astron., 7, note. The umbral structure [of a sun-spot] is quite complete, and made up of sunken banks of filaments.

1879.  Newcomb & Holden, Astron., 286. The observations consisted in measuring the relative amounts of umbral, penumbral, and photospheric radiation.

1885.  Agnes M. Clerke, Pop. Hist. Astron., 201. Vapours which are dispersed over the unbroken solar surface are accumulated in the umbral cavity.

  3.  Geol. The special designation of a series of rocks occurring in Pennsylvania.

1858.  H. D. Rogers, Geol. Pennsylv., I. 144. The Umbral Series contains, in Pennsylvania, but one formation—the Umbral Red Shale…. Its prevailing character, which is that of a dark brownish red shale and red sandstone, it steadily maintains throughout its range.

  Hence Umbrally adv.

1852.  Sylvester, in Cambr. & Dubl. Math. Jrnl., VII. 87. Express H umbrally under the form [etc.]. Ibid. (1853), in Phil. Trans., CXLIII. 429. The determinants thus umbrally represented.