RAUWOLPIA, in botany, so named in honour of Leonhard physician at Augsburg, a genus of the Pentandria Monngynia class and order. Natural or der of Contortz. Apocinex, Jussieu. Essential character : contorted ; berry succulent, two-seeded. There are four species.
RAY, in optics, a beam of light, emit ted from a radiant, or luminous body. Rays are defined, by Sir Isaac Newton, to be the least parts of light, whether suc cessive in the same line, or contemporary in several lines. For that light consists of parts of both kinds is evident„ since one may stop what comes this moment in any point, and let pass that which comes presently after: now the least light, or part of light, which maybe thus stopped, he calls a ray of light.
Rays of the Sun. It has been found by experiment, that there is a very great difference in the heating power of the different rays of light.
It appears, from the experiments of Dr. Herschel, that this heating power in creases froth the middle of the spectrum to the red ray, and is greatest beyond it, where the rays are invisible. Hence it is inferred that the rays of light and caloric nearly accompany each other, and that the latter are in different pro portions in the different coloured rays. They are easily separated from each other, as when the sun's rays are trans mitted through a transparent body, the rays of light pass on seemingly undimin ished, but the rays of caloric are inter, cepted. When the sun's rays are directed to an opaque body, the rays of light are reflected, and the rays of caloric are ab• sorbed and retained. This is the case with the light of the moon, which, however much it may be concentrated, gives no indication of being accompanied with heat. It has also been shown, that the different rays of light produce different chemical effects on the metallic Salts and oxides. These effects increase
on the opposite direction of the rum, from the heating power of the rays. From the middle of the spectrum, to wards the violet end, they become more powerful, and produce the greatest ef fect beyond the visible rays. From these discoveries it appears that the solar rays are of three kinds : 1. Rays which pro duce heat ; 2. Rays which produce co lour ; and, 3. Rays which deprive metal lic substances of their oxygen. The first set of rays is in greatest abundance, or are most powerful towards the red end of the spectrum, and are least refracted. The second set, or those which illuminate objects, are most powerful in the middle. of the spectrum. And the third set pro duce the greatest effect towards the violet end, where the rays are most re, fracted. The solar rays pass through transparent bodies without increasing their temperature. The atmosphere, for instance, receives no increase of tempe rature by transmitting the sun's rays, tili these rays are reflected from other 'no, dies, or are communicated to it by bodies which have absorbed them. This is also. proved by the sun's rays being trans-, mitted through convex lenses, producing a high degree of temperature when they are concentrated, but giving no increase of temperature to the glass itself. By, this method the heat which proceeds from the sun can be greatly increased. Indeed, the intensity of temperature pro, duced in this way is equal to that of the, hottest furnace. This is done, either by reflecting, the sun's rays from a concave polished mirror, or by concentrating or collecting them by the refractive power of convex lenses, and directing the rays concentrated on the combustible body.