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 (1912) (14591420357)

Identifier: popularguidetomi00grat (find matches)

Title: 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

Year: 1912 (1910s)

Authors: Gratacap, L. P. (Louis Pope), 1851-1917

Subjects: American Museum of Natural History. Bement Collection of Minerals Mineralogy

Publisher: New York : D. Van Nostrand

Contributing Library: Robarts - University of Toronto

Digitizing Sponsor: University of Toronto

Text Appearing Before Image:

seen. These axes are aptto be three. Some minerals, as epidote, show this property withsurprising beauty. RELATION OF CRYSTALS TO LIGHT. Reflection. When a ray of light strikes perpendicularly or atright angles upon any polished surface, as at a, b, (PI. 12, Fig. 255),it is thrown back in exactly the same line; but if it falls obliquelyit is reflected obliquely, as is shown by the arrows. The angleof rebound is equal to the angle of striking, or as it is commonlyexpressed, the angle of reflection is equal to the angle of inci-dence. Luster is an effect of reflection. Refraction. When light passes from one medium to anotherof a different density, as from air to water, it is turned out of itsstraight direction. If its course be perpendicular to the surface,as at ab (PI. 12, Fig. 256), it will not be diverted; but if it fallsobliquely, as at cd, it will be refracted and proceed to e. If this re-fracting medium has parallel surfaces, the ray on leaving it is 66 A POPULAR GUIDE TO MINERALS

Text Appearing After Image:

^ig- 259 Fig. 260 PLATE 12 again bent back, parallel to its original course, as is seen in thefigure. Double Refraction. Some substances possess the property ofsplitting the ray which passes through them, producing an effect DEFINITION OF TERMS 67 which is known as double refraction (PI. 12, Fig. 257) as one rayoutside of the substance becomes two rays (of less brilliancy)within it, and an object seen through such a doubly refractingmedium may appear doubled (PI. 12, Fig. 258). The wave theoryof light affirms that in double refraction the ray of common lightentering the doubly refracting substance is separated into two re-sulting rays, which are polarised in planes at right angles to eachother. Polarization. By the undulatory theory of light it is assumedthat in common light the undulations take place in every planethat obtains or that the light may be represented as a round rod(PI. 12, Fig. 259) the change in common light produced by polari-zation, causes the undulations to take plac

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 1912 optical phenomena in crystals book illustrations natural history geology mineralogy high resolution images from internet archive canada

1912

University of Toronto

http://commons.wikimedia.org/

public domain