EXPLANATION OF SCIENTIFIC TERMS. 



RAYS, CONVERGENT. See Reflexion, 

 Laws of. 



- DIVERGENT. See Reflexion, 

 Laws of. 



. PARALLEL. See Reflexion, 



Laws of. 



REFLEXION OF. See Re- 



flexion. 



REFRACTION OF. See Re- 

 fraction. 



ORDINARY AND EXTRAOR- 

 DINARY. See Refraction, Double. 



RE- ACT ION. See Action. 



REAUMUR'S THERMOMETER (so 

 called from the name of its inventor) is 

 that in which the space between the 

 freezing and the boiling points of water 

 is divided into eighty parts, or degrees ; 

 the freezing point being marked 0, or 

 zero, and the boiling point eighty. The 

 degrees are continued of the same size, 

 below and above these points ; those be- 

 low being reckoned negative. 



REFLEXiON (from the Latin reflectere, 

 to bend back) is a term generally used in 

 natural philosophy to denote the re- 

 bound of the rays of light, heat, or sound, 

 from an opposing surface. Polished sur- 

 faces reflect the light to the eye, and 

 are, therefore, more generally termed 

 reflectors or mirrors. Heat and sound 

 are reflected without relation to the eye, 

 and are returned from more rugged ob- 

 jects. 



LAWS OF. The reflexions 



of light, heat, and sound, are found to 

 obey the same laws as the rebound of 

 elastic balls projected upon elastic sur- 

 faces. It is, therefore, that the particles 

 of light and air have been treated as 

 being reflected by virtue of their elas- 

 ticity ; although, on this principle, we 

 cannot well account for the reflexion of 

 light from both surfaces of a glass mirror. 



ANGLE OF. The law of 



reflexion is generally expressed by the 

 assertion that " the angle of incidence is 

 always equal to the angle of reflexion," 

 and is thu$ explained : 



If ABX/Sj.ll.) be a plane surface, 

 and a ball, at D, be 

 impelled towards C, 

 perpendicular to that 

 surface, in the direc- 

 tion D C, it will re- 

 bound, from the 

 point C, back to- 

 ^ wards D, in the 

 same line C D ; but 



to the same 

 C, from any 

 E, in a direc- 

 iot perpendicular to A B, it will 

 nd 011 the other side of D C, to- 

 ll F, in such a manner that E C D, 



is termed " the angle of incidence," 



shall always be equal to F C D, " the 

 angle of reflexion" 



If, i- stead of a ball, we suppose a ray 

 of light to emanate from E, and fall 

 upon C, it will be reflected in the same 

 manner to the eye at F, along the line 

 C F ; in which direction only could an 

 object, reflected from the point C. be 

 visible A pulse of air, which is some- 

 times called a " ray of sound," follows 

 the same law, and, if proceeding from E, 

 would be reflected from C, directly in the 

 line C F, in the points of which it would 

 be heard (if sufficiently strong), as a re- 

 flected sound, or echo. Some authors 

 call E C B the angle of incidence, and 

 A C F that of reflexion ; but the mis- 

 nomer is of little consequence, for they, 

 too, are equal. 



When the reflexion is made from a 

 concave, or from a convex surface, the 

 angles of incidence and reflexion are still 

 equal ; but they are measured by the 

 tangent, or rather tangential p'ane, which 

 touches the curve at the point on which 

 the incident ray falls. By this, we shall 

 account for the several cases at pages 

 15 18 in Optics. We may add, that all 

 terrestrial rays are divergent^ as proceed- 

 ing from a point ; but those of the sun 

 are, on account of his immense distance, 

 considered as parallel. Convergent are 

 such as meet in a focus, which they can 

 only do by reflexion, or by refraction. 



REFLECTING MICROSCOPES AND 

 TELESCOPES are such as carry a mag- 

 nified image of the object to the eye, by 

 means of rays reflected from a concave 

 speculum. 



REFRACTION. When we immerse one 

 end of a rod, in a slanting direction, in a 

 vessel of water, the part immersed ap- 

 pears as if it were bent, or broken, at the 

 surface of the liquid. This phenomenon 

 is the consequence of the rays of light 

 (by which the rod is rendered visible) 

 being bent in their course ; the straight- 

 lined direction, in which they originally 

 issued, being changed (Latin refractus, 

 broken) by falling on another medium. 

 Refraction is, therefore, used in natural 

 philosophy as the denomination of that 

 deviation from its course, which a body 

 invariably experiences, when passing, in 

 an oblique direction, from one fluid 

 medium to another of a different density. 

 The term is chiefly applied to the rays of 

 light as they pass through transparent 

 bodies. 



REFRACTIVE POWER. The various 

 transparent media refract the rays of light 

 in different degrees. Let A B a b 

 (fig. 12.) be a transparent body on which 

 a ray, C D, incides from a luminous point 

 C, upon the plane surface, A B, at D. 

 Were this ray to preserve its rectilinear 

 direction, it would pass on to the point 



