REFRACTION AND POLARIZATION. 



C9 



series of rin^s are visible only when viewed in directions very 

 oblique to one another. 



4. Circular Polarization in Uniaxial Crystals. — It is stated 

 on page 53 that quartz crystals have often a left-handed and a 

 right-handed arrangement of planes. This is connected with a 

 right-handed and left-handed molecular structure, in crystals of 

 this species. When a plate cut at right angles to the axis is ex- 

 amined by the polarized light, instead of presenting a black 

 cross, the centre of the rings appears brightly colored, and if the 

 polarizer is revolved, this color changes from blue to yellow, 

 th< n red, right-handed crystals requiring revolution to the 

 right and left-handed to the left for this succession. This prop- 

 erty seems to distinguish the smallest grains of quartz, and may 

 be easily observed in a good polariscope. 



5. Anomalies in Polarization.— There are some isometric 

 crystals which have the property of polarization. Boracite is 

 one example ; and it is explained by the presence of another 

 mineral in minute particles, distributed regularly through the 

 crystals. Perofskite is another case ; and it has suggested a 

 doubt as to its being isometric. Octahedrons of alum some- 

 times have polarization, and it has 



been shown to be due to the crystals 7. 



being made up of thin plates — light, 

 when transmitted through a pile of 

 such plates, becoming polarized. Dia- 

 monds are sometimes uniaxial. 



Analcite was long since described 

 by Sir David Brewster as an example 

 of polarization under the isometric 

 system. Its trapezohedrons exhibit 

 a symmetrical arrangement of lines of 

 prismatic colors and alternating dark 

 lines with cross-bands, as imperfectly 

 shown in the annexed figure. Trapezohedrons of leucite are 

 somewhat similar in their polarizing character. The effect in 

 both species is connected with twinning ; but, besides, accord- 

 ing to recent observers, the crystallization is dimttric. One 

 writer makes crystals of analcite to be trimetric twins, analogous 

 those of phillipsite. Twinning in crystals is a very common 

 Bource of irregularities. A regular twinning of laminae of bi- 

 axial crystals around a centre may give a uniaxial character to 

 the twin. Apophyllite is a dimetric species, showing peculiari- 

 ties in its colors arising from the different action of the mineral 

 in light of different colors. 



