. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 14. Path of a ray of light through a plate with parallel sides. air, the ray is again refracted, this time away from the normal, and takes the path CF. From the geometry of Fig. 14, it is easily seen that the second angle of refraction, FCE^, is equal to the first angle of incidence, ABD; and that the paths of the ray outside the plate, namely
. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 14. Path of a ray of light through a plate with parallel sides. air, the ray is again refracted, this time away from the normal, and takes the path CF. From the geometry of Fig. 14, it is easily seen that the second angle of refraction, FCE^, is equal to the first angle of incidence, ABD; and that the paths of the ray outside the plate, namely AB and CF, are parallel. The direction of the ray on emerging from the plate is therefore the same as the original direction, but its path has been shifted a small distance, represented in Fig. 14 by B'F. On observing a small object A through a transparent body with parallel sides, it will be seen in very nearly the position it really occupies; this will not be the case however when the bounding surfaces, MN, NP (Fig. 15), are not parallel. Let the bounding surfaces, MN, NP, of the transparent body in Fig. 15 be inclined to each other at an angle MNP. We are then dealing with the path of a ray of light through a prism. As before, let the path of the ray of light incident upon MN be AB ; on entering the solid it will be bent towards the normal GH, and will take the path BD. On emerging into air, the ray will be bent away from the normal KL and will take the path DE. The angle between the original path of the ray in air and its final path is ACF, and this measures the total amount of bendins it has undergone. The source of light A, if observed through the prism, will appear not in the position it actually occupies, but at some point on the line ECF which makes with the direction AB an angle ACF. I'his angle varies under different conditions. It will be greater the greater the refracting angle MNP of the prism, and the greater the refractive index of the substance of the prism ;
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