and the Collezione paleografia Vaticana, the issue of which was
commenced in 1905. Excellent photographic reproductions on a
reduced scale are being issued by the British Museum and by the
Bibliothèque Nationale in Paris. (E. M. T.)
ILLUMINATI (Lat. illuminare), a designation in use from the
15th century, and applied to, or assumed by, enthusiasts of
types distinct from each other, according as the “light” claimed
was viewed as directly communicated from a higher source, or as
due to a clarified and exalted condition of the human intelligence.
To the former class belong the alumbrados of Spain. Menendez
Pelayo first finds the name about 1492 (in the form aluminados,
1498), but traces them back to a Gnostic origin, and thinks their
views were promoted in Spain through influences from Italy.
One of their earliest leaders, born in Salamanca, a labourer’s
daughter, known as La Beata de Piedrahita, came under the
notice of the Inquisition in 1511, as claiming to hold colloquies
with our Lord and the Virgin; having high patrons, no decision
was taken against her (Los Heterodoxos Españoles, 1881, lib. v.).
Ignatius Loyola, while studying at Salamanca (1527) was brought
before an ecclesiastical commission on a charge of sympathy
with the alumbrados, but escaped with an admonition. Others
were not so fortunate. In 1529 a congregation of unlettered
adherents at Toledo was visited with scourging and imprisonment.
Greater rigours followed, and for about a century the
alumbrados afforded many victims to the Inquisition, especially
at Cordova. The movement (under the name of Illuminés)
seems to have reached France from Seville in 1623, and attained
some proportions in Picardy when joined (1634) by Pierre
Guérin, curé of Saint-Georges de Roye, whose followers, known
as Guérinets, were suppressed in 1635 (Hermant, Hist. des
hérésies, 1717). Another and obscure body of Illuminés came
to light in the south of France in 1722, and appears to have
lingered till 1794, having affinities with those known contemporaneously
in this country as “French Prophets,” an offshoot
of the Camisards. Of different class were the so-called Illuminati,
better known as Rosicrucians, who claimed to originate in 1422,
but rose into notice in 1537; a secret society, combining with
the mysteries of alchemy the possession of esoteric principles
of religion. Their positions are embodied in three anonymous
treatises of 1614 (Richard et Giraud, Dict. de la théol. cath.).
A short-lived movement of republican freethought, to whose
adherents the name Illuminati was given, was founded on
May-day 1776 by Adam Weishaupt (d. 1830), professor of
Canon Law at Ingolstadt, an ex-Jesuit. The chosen title of
this Order or Society was Perfectibilists (Perfektibilisten). Its
members, pledged to obedience to their superiors, were divided
into three main classes; the first including “novices,”
“minervals” and “lesser illuminati”; the second consisting
of freemasons, “ordinary,” “Scottish” and “Scottish knights”;
the third or “mystery” class comprising two grades of “priest”
and “regent” and of “magus” and “king.” Relations with
masonic lodges were established at Munich and Freising in 1780.
The order had its branches in most countries of the European
continent, but its total numbers never seem to have exceeded
two thousand. The scheme had its attraction for literary men,
such as Goethe and Herder, and even for the reigning dukes
of Gotha and Weimar. Internal rupture preceded its downfall,
which was effected by an edict of the Bavarian government
in 1785. Later, the title Illuminati was given to the French
Martinists, founded in 1754 by Martinez Pasqualis, and to their
imitators, the Russian Martinists, headed about 1790 by Professor
Schwartz of Moscow; both were Cabalists and allegorists,
imbibing ideas from Jakob Boehme and Emmanuel Swedenborg
(Bergier, Dict. de théol.).
See (especially for details of the movement of Weishaupt,) P. Tschackert, in Hauck’s Realencyklopädie (1901). (A. Go.*)
ILLUMINATION, in optics, the intensity of the light falling
upon a surface. The measurement of the illumination is termed
photometry (q.v.). The fundamental law of illumination is
that if the medium be transparent the intensity of illumination
which a luminous point can produce on a surface directly exposed
to it is inversely as the square of the distance. The word transparent
implies that no light is absorbed or stopped. Whatever,
therefore, leaves the source of light must in succession pass
through each of a series of spherical surfaces described round
the source as centre. The same amount of light falls perpendicularly
on all these surfaces in succession. The amount received
in a given time by a unit of surface on each is therefore inversely
as the number of such units in each. But the surfaces of spheres
are as the squares of their radii,—whence the proposition.
(We assume here that the velocity of light is constant, and
that the source gives out its light uniformly.) When the rays
fall otherwise than perpendicularly on the surface, the illumination
produced is proportional to the cosine of the angle of
obliquity; for the area seen under a given spherical angle
increases as the secant of the obliquity, the distance remaining
the same.
As a corollary to this we have the further proposition that the apparent brightness of a luminous surface (seen through a transparent homogeneous medium) is the same at all distances.
The word brightness is here taken as a measure of the amount of light falling on the pupil per unit of spherical angle subtended by the luminous surface. The spherical angle subtended by any small surface whose plane is at right angles to the line of sight is inversely as the square of the distance. So also is the light received from it. Hence the brightness is the same at all distances.
The word brightness is often used (even scientifically) in another sense from that just defined. Thus we speak of a bright star, of the question—When is Venus at its brightest? &c. Strictly, such expressions are not defensible except for sources of light which (like a star) have no apparent surface, so that we cannot tell from what amount of spherical angle their light appears to come. In that case the spherical angle is, for want of knowledge, assumed to be the same for all, and therefore the brightness of each is now estimated in terms of the whole quantity of light we receive from it.
The function of a telescope is to increase the “apparent magnitude” of distant objects; it does not increase the “apparent brightness.” If we put out of account the loss of light by reflection at glass surfaces (or by imperfect reflection at metallic surfaces) and by absorption, and suppose that the magnifying power does not exceed the ratio of the aperture of the object-glass to that of the pupil, under which condition the pupil will be filled with light, we may say that the “apparent brightness” is absolutely unchanged by the use of a telescope. In this statement, however, two reservations must be admitted. If the object under examination, like a fixed star, have no sensible apparent magnitude, the conception of “apparent brightness” is altogether inapplicable, and we are concerned only with the total quantity of light reaching the eye. Again, it is found that the visibility of an object seen against a black background depends not only upon the “apparent brightness” but also upon the apparent magnitude. If two or three crosses of different sizes be cut out of the same piece of white paper, and be erected against a black background on the further side of a nearly dark room, the smaller ones become invisible in a light still sufficient to show the larger. Under these circumstances a suitable telescope may of course bring also the smaller objects into view. The explanation is probably to be sought in imperfect action of the lens of the eye when the pupil is dilated to the utmost. Lord Rayleigh found that in a nearly dark room he became distinctly short-sighted, a defect of which there is no trace whatever in a moderate light. If this view be correct, the brightness of the image on the retina is really less in the case of a small than in the case of a large object, although the so-called apparent brightnesses may be the same. However this may be, the utility of a night-glass is beyond dispute.
The general law that (apart from the accidental losses mentioned above) the “apparent brightness” depends only upon the area of the pupil filled with light, though often ill understood, has been established for a long time, as the following quotation from Smith’s Optics (Cambridge, 1738), p. 113, will show:—
