LIGHT

LIGHT, 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, $\backslash frac\{1\}\{10\}$th 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 $\backslash frac\{1\}\{10\}$th 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 an intermediate thickness and distinctness 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 $\backslash frac\{1\}\{10\}$th 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 Grimaldo 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, and

Limbers. and that those which are least refrangible are most inflexible." From other experiments, he concludes, that the most inflexible rays are also most deflexible. In the sequel of his paper, he ascertains the proportion which the angle of inflection bears to that of deflection at equal incidences, and the proportion which the different flexibilities of the different rays bear to one another. We shall give an account of some other experiments made by him, and of the inferences drawn from them, under the word REFLEXITY, to which a reference has already been made.