I 



THE PROPERTIES OF MINERALS. 685 



are tlie result of interference. The last model shows the effect of inter- 

 ference in monochromatic or homogeneous light. With common light 

 the prismatic colors are shown in succession. 



I'OLARIZATION OK LIGHT. 



" Polarization is, in general, that change in the character of reflected 

 or transmitted light which diminishes its power of being further reflected 

 or transmitted." To follow up the mechanical notion of the nature of 

 polarized light it is necessary to refer to the wave model of common 

 light, and by separating the two parts one from the other it maj' be 

 shown how a wave of common light is reducible to two primary waves 

 whose vibrations take place in a single })lane only, or are polarized. 



Light may be polarized by reflection and simple refraction, double 

 refraction, and absorption. The first model shows a mechanical con- 

 ception of a ray of common light made up of the transversal vibrations 

 A and !>. The second model is a wire bent to represent a horizontal 

 vibration which, if kept in that position, Avill pass only through a hori- 

 zontal aperture; the third model is a wire bent to represent a per- 

 pendicular vibration which will pass only through a perpendicular 

 aperture. The next model illustrates polarization by means of reflec- 

 tion and single refraction; A- A is a model of a bundle of glass plates 

 placed at an angle of 56° 45' ; B is a ray of common light; C shows the 

 light polarized by reflection and D by refraction. 



Polarization by double refraction is shown by a glass model represent- 

 ing a rhomb of calcite. A ray of light, A, entering the rhomb is doubly 

 refracted, or divided into two rays of polarized light. One of these 

 rays, O, conforms with the law of ordinary refraction, and is called the 

 ordinary ray ; the other does not conform with the law, and is called the 

 extraordinary ray E. 



Polarization by absorption is shown by the next model, in which a 

 slice of tourmaline is regarded, mechanically, as being like a grating 

 through which polarized light may iiass, then A is the model of a slice 

 of tourmaline into which the transversal vibrations, B, are passing; the 

 horizontal wave is absorbed and the vertical polarized one passes to the 

 second slice of tourmaline, 0, where the bars (the axes) being at right 

 angles to those of A, it is stopped, and can pass through only when the 

 bars of C are parallel to those of A. 



INTERFERENCE FIGURES. 



If a section of a doubly refracting mineral, which is cut perpendicular 

 to an oi)tic axis, be examined in polarized light it will exhibit, under 

 certain conditions, coloreil rings or bands. According to the undnla- 

 tory theory of light tiiese rings arise from the interference of the waves 

 of the ordinary and extraordinary rays. 



The observation of the systems of rings, or interference figures, which 

 tliiii jdates of crystals give in polarized light affords a ready means of 



