it has been observed in the article CHEMISTRY, n° 319. (Suppl.), consists of rays differently flexible. This was established by some well devised experiments made by Henry Brougham, Esq.; of which it may be proper to give an account here.
In the first experiment, he darkened his chamber in the usual way, and let a beam of the sun's light into it through the hole of a metal plate fixed in the shutter of the window, 1/6th of an inch in diameter. At the hole within the room he placed a prism of glass, of which the refracting angle was 45 degrees, and which was everywhere covered with black paper, except a small part on each side; and through this part the light was refracted so as to form a distinct spectrum on a chart at six feet distance from the window. In the rays, at two feet from the prism, he placed a black unpolished pin, of which the diameter was 1/6th of an inch, parallel to the chart, and in a vertical position. The shadow of the pin was found in the spectrum; and this shadow had a considerable penumbra, which was broadest and most distinct in the violet part, narrowest and most confused in the red, and of intermediate thicknesses and distinctions in the intermediate colours. The penumbra was bounded by curvilinear sides, convex towards the axis to which they approached as to an asymptote, so as to be nearest to it in the place of the least refrangible rays. By moving the prism on its axis, and causing the colours to ascend and descend on any bodies that were used instead of the pin, the red, wherever they fell, made the least, and the violet the greatest shadow.
In the next experiment, a screen was substituted in the place of the pin; and this screen had a large hole, on which was a brass plate, pierced with a small hole 1/6th of an inch in diameter. While an assistant moved the prism slowly on its axis, the author observed the round image made by the different rays passing through the hole to the chart; that made by the red was greatest, that of the violet least, and that of each intermediate rays was of an intermediate size. When the sharp blade of a knife was held at the back of the hole, "so as to produce the fringes mentioned by Grimaldi and Newton, these fringes in the red were broadest and most moved inwards to the shadow, and most dilated when the knife was moved over the hole; and the hole itself on the chart was more dilated during the motion when illuminated by the red than when illuminated by any other of the rays, and least of all when illuminated by the violet."
From these two experiments, the author infers "that the rays of the sun's light differ in degree of flexibility,
(a) As dry lacmus is much cheaper than moist, it may be readily supposed that it is adulterated with sand and other substances. Valentinii Historia simplicium. Francisci ad Moen. 1716, fol. p. 152. LOT LOW
LIMBERS, in artillery, a fort of advanced train, joined to the carriage of a cannon on a march. It is composed of two shafts, wide enough to receive a horse between them, called the fillet horse; these shafts are joined by two bars of wood, and a bolt of iron at one end, and mounted on a pair of rather small wheels. Upon the axle-tree rises a strong iron spike, which is put into a hole in the hinder part of the train of the gun-carriage, to draw it by. But when a gun is in action, the limbers are taken off, and run out behind it.
LIMIT OF A PLANET, has been sometimes used for its greatest heliocentric latitude.
LAMBERT'S Problem denotes a problem that has but one solution, or some determinate number of solutions; as to describe a circle through three given points that do not lie in a right line, which is limited to one solution only; to divide a parallelogram into two equal parts by a line parallel to one side, which admits of two solutions, according as the line is parallel to the length or breadth of the parallelogram; or to divide a triangle in any ratio by a line parallel to one side, which is limited to three solutions, as the line may be parallel to any of the three sides.
LOCAL Problem, is one that is capable of an infinite number of different solutions; because the point, which is to solve the problem, may be indifferently taken within a certain extent; as suppose any where in such a line, within such a plane figure, &c. which is called a geometrical Locus.
A local problem is simple, when the point sought is in a right line; plane, when the point sought is in the circumference of a circle; solid, when it is in the circumference of a conic section; or surfolded, when the point is in the perimeter of a line of a higher kind.