Elements of natural philosophy (Volume 2-3) . al, is called the angleof refraction. How these angles § 12. These angles are always estimated from that part are estimated; a ,-1 i -i •, ot the normal drawn through the point of incidence ofthe ray, which lies in the medium of the incident wave. ELEMENTS OF OPTICS. 173 They are accounted positive when on the same side of when positivethe normal as the incident ray, and negative when ona Fig. 5. Illustration. negative; the opposite side. Thus,the angle of incidenceP D £, is always positive,as also the angle of re-fraction PDSm, whilethe angle of r
Elements of natural philosophy (Volume 2-3) . al, is called the angleof refraction. How these angles § 12. These angles are always estimated from that part are estimated; a ,-1 i -i •, ot the normal drawn through the point of incidence ofthe ray, which lies in the medium of the incident wave. ELEMENTS OF OPTICS. 173 They are accounted positive when on the same side of when positivethe normal as the incident ray, and negative when ona Fig. 5. Illustration. negative; the opposite side. Thus,the angle of incidenceP D £, is always positive,as also the angle of re-fraction PDSm, whilethe angle of reflexionPDSr, will always benegative, as it should be,since the velocity of thereflected light must becounted negative, thereflected wave being dri-ven back from the de-viating surface. § 13. When the deviating surface is curved, we con-^^ eNiatinsceive a tangent plane drawn to it at the point of incidence, .and treat this plane as the deviating surface for thatportion of the wave which is incident immediately aboutthe tangential § 14. The angle whichany ray after deviation,makes with the prolonga-tion of the same ray be-fore incidence, is calledthe deviation. Thus,Slv- D £, is the devia-tion by reflexion; andS D S1V; the deviationby refraction.
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