REFLECTION 467 REFLEX NERVOUS ACTION to one another — a proposition of great utility in lighthouse work, search-lights, etc, (2) If the paraboloid mirror be convex, parallel incident rays have, after reflection, the same course as if they had come from the geometrical focus of the paraboloid. (3) In a concave ellipsoid mirror, light diverging from one "focus" of the ellipsoid is reflected so as to con- verge on the other "focus" of the curved surface; and by a convex ellipsoidal mir- ror light converging toward the one focus is made to diverge as if it had come directly from the other focus. (4) In a hyperboloid reflector the two geometrical foci have properties corresponding to those of the ellipsoid. (5) In spherical reflectors, which are those most easily made, there is no accurate focus except for rays proceeding from the center and returning to it. When parallel rays are incident on a concave spherical mirror we see that if they be parallel to the axis of the mirror each ray is made to pass after reflection through a point, which is nearer to a point midway be- tween the mirror and its center, the narrower is the pencil of rays. If there- fore, the pencil of rays be very narrow in comparison with the radius, the rays will, after reflection, approximately con- verge on the midway point, which is called the principal focus of the mirror. The reflected rays from the various parts of the mirror form by their intersection a caustic, the apex or cusp of which is at the midway point. As to the quality of the light reflected there are some peculiarities to be ob- served. From the surface of a trans- parent body, of greater optical density than the sun-ounding medium, light polarized in the plane of incidence and reflection is more largely reflected at oblique incidences than light polarized at right angles to that plane; when the angle of incidence is such that the re- flected and refracted rays tend to be at right angles to one another, the whole of the light reflected is polarized in the plane incidence and reflection; and if light polarized at right angles to that plane be made to fall on glass at the particular angle of incidence just re- ferred to, it will not be reflected at all, but will wholly enter the glass. Plane- polarized light polarized in any other plane than that of incidence or one at right angles to it, is, after total reflec- tion in glass, found to be elliptically po- larized; and this phenomenon is always presented in reflection from metals. In the case of electro-magnetic radiation theory and practice concur in indicating that conductors are good while non-con- ductors are bad reflectors; and the same general proposition holds good with ref- erence to those more frequent but other- wise similar ether oscillations to which the phenomena of radiant heat, light, and actinism are due. REFLECTION OBSERVATIONS, in astronomy, those which measure the direction of a beam of light which has been reflected from the surface of some liquid, generally mercury. Sometimes it is a beam from a heavenly body, as in sextant observations with an artificial horizon, or in measuring the reflected zenith distance of a star with a meridian circle in an observatory, and sometimes it is a beam of light, or rather the want of light, which makes the shadow of the wires of a transit or meridian circle from a lamp used in the nadir observa- tions of an observatory. REFLECTOR, that which reflects, or throws back rays of light, heat, etc.; a reflecting surface. In optics, a device by which the rays proceeding from a luminous or heated object are thrown back or diverted in a given direction. The reflecting surface may be either plane or curved. In practice it is often made spherical or parabolic. A mirror is a familiar example of a plane re- flector. The material should be as smooth and highly polished as possible. Sheet tin is frequently used for common purposes, as for door, hall, or vehicle lamps, while for other purposes a more perfectly reflecting surface is em- ployed, such as speculum metal or silver protected by glass. Silver is the most perfectly reflecting substance known, absorbing but 9 per cent, of the incident rays, while speculum metal absorbs 37 per cent. Glass itself, owing to its prop- erty of totally reflecting incident rays at a low angle, is used in certain cases. Reflectors with parabolic surfaces are employed for throwing the light emanat- ing from objects placed in their foci in parallel straight lines to a great dis- tance, and for converging the heat rays from a distant object, as the sun, to a focus, and also, in connection with eye glasses, in the reflecting telescope, which is itself often simply denominated a reflector. REFLEX NERVOUS ACTION, in physiology, those actions of _ the_ nervous system whereby an impression is trans- mitted along sensory nerves to a nerve center, from which again it is reflected to a motor nerve, and so calls into play some muscle whereby movements are pro- duced. These actions are performed in- voluntarily, and often unconsciously, as the contraction of the pupil of the eye when exposed to strong light. See Nerve.
