EXPLANATION OF SCIENTIFIC TERMS. 



17 



Fig. 12. 



E : but, meeting with another medium 

 at the point D, it is reflected to F ; so 

 that the lumi- 

 nous point, C, 

 would be seen 

 by the eye at 

 F, and not at 

 E. Draw the 

 right line, GD 

 H, perpendi- 

 cular to A B, 

 through the a 

 point of inci- 

 dence D, which 



line, G D H, is termed the Axis of Re- 

 fraction. The angle, C D G, is the 

 angle of incidence, and H D F is the 

 angle of refraction. These angles have 

 a fixed relation to each other, in the same 

 transparent body, whatever the angle of 

 incidence may be, but that proportion is 

 different in some bodies from what it is 

 in others ; and hence those bodies are 

 said to have a greater or a less refrac- 

 tive power. When the incident ray is 

 perpendicular to the surface on which it 

 falls, (as in the direction G D H,) there 

 is no refraction. In every other direc- 

 tion, the ratio between the sines of the 

 angles of incidence and of refraction is 

 constant, and is termed the Index of Re- 

 fraction. Thus, in water, if the sine of 

 the angle of refraction be taken as unity, 

 that of incidence will be about li ; and, 

 therefore, the index of refraction in water 

 is marked in the comparative Tables as 

 being 1.336. This is called the Law of 

 the Sines. See Angle. 



In the passage of a ray of light from a 

 dense to a rarer medium, the refraction 

 is reversed. The angle of incidence (at 

 the point D, where the two media meet) 

 is then the lesser one ; and a luminous 

 object at F, would be seen by an eye at 

 C. See Tangent. 



REFRACTION, DOUBLE. The refrac- 

 tion last mentioned is now termed Simple 

 Refraction, because a theory has been 

 formed of the laws of that Double Refrac- 

 tion, which was long ago observed in Ice- 

 land spar, but has since been found to 

 take place in many other crystals, and 

 may be even artificially communicated to 

 glass : the ray, or pencil of light, when 

 falling on a crystal of double refraction, 

 produces a double image. It separates 

 into two parts or rays; one of which 

 follows the ordinary law of the sines, and 

 the other is refracted u according to a 

 new, or extraordinary law." These two 

 pencils of light, into which the ray is 

 divided, are termed by Dr. Brewster the 

 ordinary ray, and the extraordinary ray. 



REFRACTING MICROSCOPES AND 

 TELESCOPES are such as show a mag- 

 nified image of an object, by means of 

 rays of light refracted and collected into 

 a focus through lenses. 



REFRANGIBILITY is the capability of 

 being refracted, and has been employed to 

 designate the degree of that property 

 which is possessed by the several divisions 

 of a ray of light. It is owing to their 

 various refrangibilities that the threads 

 or rays separate from each other in passing 

 through the prism, and thereby form the 

 coloured spectrum. 



RELATIVE GRAVITY. See Gravity. 

 REPULSION (Latin repellere, to drive 

 back) is the name of a power or principle 

 in the particles of natural bodies, by 

 which, under certain circumstances, they 

 refuse to meet one another. It is the 

 opposite of attraction, and equally inex- 

 plicable. The elasticity of bodies has 

 been referred to this principle, which is 

 merely giving us another name for an 

 unknown cause. 



RESISTING FORCE._See Force. 

 RESULTANT. See Forces, Composition 



of. 



RHOMBUS. A rhombus is a surface hav- 

 ing four equal sides, but of which the 

 angles are unequal ; it is a square pressed 

 out of shape until it assumes the form of 

 the diamond of a pack of cards. If the op. 

 posite sides only are equal, it is called a 

 rhomboid or rhomboides. It is a compressed 

 parallelogram, its opposite angles only 

 being equal. In describing crystals, some 

 are termed rhombs or rhomboids, because 

 they are solids whose faces have those 

 figures. They are rhomboidal solids. 

 Others are described by the number of 

 their sides (Greek hedrd) or faces. Thus 

 a solid with four faces is called a tetra- 

 hedron ; with six, a hexahedron ; with 

 eight, an octohedron ; with twelve, a 

 dodecahedron , with twenty, an icosahe- 

 dron ; and, generally, a "solid having 

 many sides, is a polyhedron. All these 

 have their adjectives tetrahedral, hexahe- 

 dral, &c. equivalent to the English four- 

 sided, six-sided, &c. A cube is a hexa- 

 hedron. 



RIGHT ANGLE. See Angle. 

 RIGHT LINE. The same as a straight 



line See Curve. 



RIGIDITY OF CORDAGE. One of the 

 properties, which is useful in ropes and 

 cordage, when applied to machinery, is 

 flexibility, so that they may be easily bent, 

 and apply easily to wheels and pulleys. 

 The opposite of this property is termed 

 rigidity. 



ROCHE LLE SALT is a chemical prepa- 

 ration, used in medicine ; the tartrate of 

 potash and soda. It crystallizes in large 

 regular eight-sided prisms. 

 ROCK-CRYSTAL. See Quartz. 

 RUBY. See Corundum. 

 SAFETY-VALVE (a necessary appendage 

 to a steam-engine) is a valve opening out- 

 wards from a boiler, and loailed with a 

 weight sufficient to withstand the elastic 

 pressure of the steam until it rise to a 



