A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . rrelations to the axes, and these are expressed in the so-called indicesor parameters, which mean relative points of intersection, by thefaces, of the axes of the crystal and such intercepts are related toeach other by a simple law, that the smaller divided into the largerones give simple rational numbers or infinity or zero. In other wordsthe axes are cut by the faces at distances from the center of the crys-tal, (the point where the a
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . rrelations to the axes, and these are expressed in the so-called indicesor parameters, which mean relative points of intersection, by thefaces, of the axes of the crystal and such intercepts are related toeach other by a simple law, that the smaller divided into the largerones give simple rational numbers or infinity or zero. In other wordsthe axes are cut by the faces at distances from the center of the crys-tal, (the point where the axes intersect), which are related to eachother as ^, I, 2, 3, etc., or, a face may be parallel to an axis, whensuch an intercept becomes infinity. The ratio of the axial intercepts is alone important. Ofcourse all crystals of one mineral are to be referred to one andthe same system. The various examples of crystals in the samespecies of minerals, while different between themselves, present newcrystallographic forms of the same system and all will be found in-cluded under one systematic law. The angles between crystal faces 86 A POPULAR GUIDE TO MINERALS. Fig. A are determined by an instru-ment called a Goniometer,(angle-measure), and theprinciple involved is thecatching of a reflection(signal) from the center orpole of one face, and then,by turning the crystal, acompanion reflection fromthe center or pole of the ad-joining face, whose anglewith the first it is desirjed toknow. The angle thus fixedwill really be the supple-ments of the actual inter-facial angles. Thus in the angle AOD, meas-ured by the goniometer, isthe supplement of the angle between the faces a and drawings are not true perspective drawings. Theyare drawn as if the eye and the crystal were at an infinite distancefrom each other, or, as Prcf. Lewis expresses it: the crystal isdrawn in much the same way as it would appear, if viewed througha telescope adjusted for a very distant object.^ There a
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
