RADIATION. The passage of light or heat in rays, or straight lines; these being projected from every luminous, or heated point, in all directions. RADIUS. The distance from the centre of a circle, to its circumference; or one half of its diameter. In the plural denominated radii. RAINBOW. An appearance in the atmosphere, occasioned by the decomposition of solar light, in its refraction, and reflection, in passing through drops of rain. The bow can be seen, only when the sun is near the horizon, when the back is turned towards it, and there is a shower in the opposite direction. RAY. A single line of light, emitted in one direction, from any luminous point. REACTION. Every body, whether in a state of motion, or at rest, tends to remain in such state, and resists the action of any other body upon it, with a force equal to that action. This resistance, is called its _reaction_. RECEIVER. This name is applied to glass vessels of various kinds, appertaining to the air pump, and from which the air may be exhausted. They are made to contain, or receive, any article upon which an effect is to be produced, by taking off the pressure of the atmosphere. REFRACTION, of the rays of light, is the bending of those rays, when they pass obliquely from one medium into another of different density. A stick held obliquely in water, appears bent or broken at the surface of the fluid. REFRANGIBILITY. Capacity of being refracted. Light is decomposed by the prism, because its component parts are refrangible in different degrees, by the same refracting medium. REPULSION. The reverse of attraction. A tendency in particles, or in masses of matter, to recede from each other. The matter of heat within a body, appears to counteract the attraction of its particles, so as to prevent absolute contact. RETINA. That part of the ball of the eye, upon which the images of visible objects are formed; and from which, the idea of such forms, is conveyed to the mind. REVOLUTION, of a planet; is either diurnal, or annual; the former, is its turning upon its own axis; the latter, is its passage in its orbit. SATELLITES. Moons, secondary planets. SEGMENT OF A CIRCLE. A portion, or part of a circle; called also, an arc of a circle. SEMI-DIAMETER. Half the diameter. The semi-diameter of the earth, is the distance from its surface, to its centre. SIDERIAL. Belonging to the stars. A siderial day, is the time required for a star to reappear on a given meridian. A siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated.