. 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. 17. Dispersion of light by a plate with parallel sides. bent out of their course least, the violet rays most, and the other rays will fall in their proper order between the two extreme*. In Fig. 16 the ray of white light AB falls upon the surface MN of a refracting substance. Owing to the different refrangibilities of the constituents of the r
. 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. 17. Dispersion of light by a plate with parallel sides. bent out of their course least, the violet rays most, and the other rays will fall in their proper order between the two extreme*. In Fig. 16 the ray of white light AB falls upon the surface MN of a refracting substance. Owing to the different refrangibilities of the constituents of the ray, these latter are separated and we get the original single ray of white light split up into red {R), yellow (G), green {Gr), blue {Bl), and violet (F), rays, deviating from each other slightly in direction. Between the rays of the colours just mentioned lie rays of intermediate tints. The decomposition of white light into its coloured con- stituents, or the dispersion of light, varies according to the dispersive power of the refracting substance. It is the more distinct the greater the angle between the extreme red ray and the extreme violet ray. We have now to consider the dispersion of the light which passes through a precious stone. We will take first the case in which the stone has the form of a plate with parallel sides, as in Fig. 17, and afterv\ards the case in which these bounding surfaces are inclined to each other and so form a prism. The ray of white light, AB, falling obliquely on the surface MN of the precious stone, is split up into the differently coloured rays lettered BR, BG, BGr, BBl, BV. These rays pass out of the precious stone at the surface PQ in the directions RR', VV, &c., all being parallel to the original white ray AB, as was explained before in connection with Fig. 14. The eye placed at R'V will receive all these differently coloured rays at the same time and in the same direction ; the effect of this will be to produce in the eye the sensation
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