Diamonds and precious stones, a popular account of gems .. . dium, its course is in a straightline, as shown in Fig. i6, a phenomenon with whicheveryone is familiar. But when it passes from onemedium into another, the case is generally different,and the ray suffers a remarkable modification. Itis then more or less diverted from its primitivedirection, and has the appearance of being broken,whence the phenomenon has been termed stick plunged into water will exemplify thiseffect. The extent to which the luminous rays arediverted in traversing transparent bodies variesgreatly. This v
Diamonds and precious stones, a popular account of gems .. . dium, its course is in a straightline, as shown in Fig. i6, a phenomenon with whicheveryone is familiar. But when it passes from onemedium into another, the case is generally different,and the ray suffers a remarkable modification. Itis then more or less diverted from its primitivedirection, and has the appearance of being broken,whence the phenomenon has been termed stick plunged into water will exemplify thiseffect. The extent to which the luminous rays arediverted in traversing transparent bodies variesgreatly. This variation is generally connected withdifferences in the nature and composition of the re-fracting bodies; but it is likewise intimately con-nected, as experiments prove, with the molecular 20 PRECIOUS STONES. constitution of these bodies. For example, Icelandspar and aragonite, whose chemical compositionis identical, both consisting of pure carbonate oflime, refract the light unequally, for the sole reasonthat their molecular constitution is very Fig. i6.—Course of a luminous ray in a homogeneous medium. Double Refraction.—Among diaphanous bodiesthere is a numerous class of substances that possessthe curious quality of presenting two images of oneobject. If a crystal of Iceland spar is placed upona piece of white paper bearing an inscription, as inFig. 17, two images will be visible of every point, OPTICAL PROPERTIES. 21 and both images will show deviation. This is aninstance of what is called double refraction. When a body, crystallized or not, is perfectlyhomogeneous in all its parts, so that its elementsare disposed everywhere in a uniform manner, onecan easily understand that the light must traverseit regularly, and must present a single image ofevery object: such bodies possess the property ofsimple refraction.
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectgems, booksubjectprec
