ts. The sign of summation [Sigma] is used in
cases where there are several absorption bands, and consequently
several similar terms on the right-hand side, each with a different
value of [lambda]_m. This would occur if there were several kinds of
ions, each with its own natural period.

In a region where there is no absorption, we have [kappa] = 0 and
therefore g = 0, and we have only one equation, namely,

____
\ D[lambda]^2
n^2 = 1 + > --------------------------,
/___ ([lambda]^2 - [lambda]_m^2)

which is identical with Sellmeier's result. As [lambda]_m, is a
wave-length corresponding to an absorption band, this formula can be
used to find values of [lambda]_m which satisfy the observed values
of n within the region of transparency, and so to determine where the
absorption bands are situated. In this way the existence of bands in
the infrared part of the spectrum has been predicted in the case of
quartz and detected by experiments on the selective reflection of the
material.

_References._--For the theory of dispersion see P. Drude, _Theory of
Optics_ (Eng. trans.); R. W. Wood, _Physical Optics_; and A. Schuster,
_Theory of Optics_. For descriptive accounts, see Wood's _Physical
Optics_, T. Preston's _Theory of Light_, E. Edser's _Light_. The last
work contains an elementary treatment of Sellmeier's theory.
(J. R. C.)

D'ISRAELI (or DISRAELI), ISAAC (1766-1848), English man of letters,
father of the earl of Beaconsfield (q.v.), was born at Enfield in May
1766. He belonged to a Jewish family which, having been driven by the
Inquisition from Spain, towards the end of the 15th century, settled as
merchants at Venice, and assumed the name which has become famous; it
was generally spelt D'Israeli until the middle of the 19th century. In
1748 his father, Benjamin D'Israeli, then only about eighteen years of
age, removed to England, where, before passing the prime of life, he
amassed a competent fortune, and retired from business. He belonged to
the London congregation of Spanish and Portuguese Jews, of which his son
also remained a nominal member until after Benjamin D'Israeli died at
the end of 1816.

The strongly marked characteristics which determined Isaac D'Israeli's
career were displayed to a singular degree even in his boyhood. He spent
his time over books and in long day-dreams, and evinced the st