A Mathematical and Philosphical Dictionary
, something that partakes of the nature of an accident; or that is indifferent, or not essential to its subject.—Thus whiteness is accidental to marble, and sensible heat to iron.
Accidental Colours, so called by M. Buffon, are those which depend on the affections of the eye, in contradistinction to such as belong to light itself.
The impressions made upon the eye, by looking stedfastly on objects of a particular colour, are various according to the single colour, or assemblage of colours, in the object; and they continue for some time after the eye is withdrawn, and give a false colouring to other objects that are viewed during their continuance. M. Buffon has endeavoured to trace the connection between these accidental colours, and those that are natural, in a variety of instances. M. d' Arcy contrived a machine for measuring the duration of those impressions on the eye; and from the result of several trials he inserred, that the effect of the action of light on the eye continued about eight thirds of a minute.
The subject has also been considered by M. de la Hire, and M. Aepinus, &c. See Mem. Acad. Paris 1743, and 1765; Nov. Com. Petrop. vol. 10; also Dr. Priestley's Hist. of Discoveries relating to Vision, pa. 631.
Accidental Point, in Perspective, is the point in which a right line drawn from the eye, parallel to another right line, cuts the picture or perspective plane.
Let AB be the line given to be put into perspective, CFD the picture or perspective plane, and E the eye: draw EF parallel to AB; so shall F be the accidental point of the line AB, and indeed of all lines parallel to it, since only one parallel to them, namely EF, can be drawn from the same point E: and in the accidental point concur or meet the representations of all the parallels to AB, when produced.
It is called the accidental point, to distinguish it from the principal point, or point of view, where a line drawn from the eye perpendicular to the perspective plane, meets this plane, and which is the accidental point to all lines that are perpendicular to the same plane.
Accidental Dignities, and Debilities, in Astrology, are certain casual dispositions, and affections, of the planets, by which they are supposed to be either strengthened, or weakened, by being in such a house of the figure.