140 



POLARIZING STRUCTURE ARTIFICIALLY PRODUCED. 



] eccive upon the eye tlie rays reflected from the second surface and emergent 

 from the first, the reflecting surface itself forms an analyzer sufficiently perfect 

 to exhibit the rings. But as the angle of reflection is not truly the polarizing 

 angle, when the crystal is in azimuth 90° the dark baud will not be as large as 

 is the case in the rings seen with a better analyzer by transmitted light. Fig. 

 24 exhibits the appearance of these reflected rings. 



In Figs. 25 and 26, which follow, are seen the appearances presented when 

 the subject of examination is saltpetre, (nitrate of potash,) in which the axes 

 are inclined to each other 6°. The plane of the axes of the crystal being brought 



w 





Fis. 25. 



Fi-. 26. 



into coincidence with the plane of polarization of the incident light, and the 

 analyzer being crossed upon the polarizer, a system of lemniscate curves is 

 seen, like that shown in Fig. 25, intersected by a dark cross, of which the bar 

 coinciding in direction with the plane of the axes is longest. If the analyzer 

 be turned 90°, the colors become complementary, and the cross becomes white; 

 but if, the analyzer and polarizer remaining fixed, the crystal itself is turned in 

 azimuth, the cross will break at the centre, forming two curves, which, when 

 the rotation becomes forty-five degrees, assume the form of two opposite hyper- 

 bolas. This appearance is exhibited in Fig. 26. 



In the prosecution of his investigations, Sir David Brewster arrived at the 

 discovery that the polarizing structure could be artificially produced in glass by 

 heat or by rapid cooling; that this cfiect is transient when the heat is below 

 the point of softening or fusing the substance ; but that when it is carried beyond 

 that point, and cooling rapidly follows, as in glass which is not annealed, the 

 structure is permanent. He found that the same structure could be produced 

 by pressure, by torsion, by tension, or by flexure; and traced the transient con- 

 dition of the same kind produced by heat to the mechanical effects of unequal 

 expansion. Any solid transparent substance, organic or mineral, was found by 

 him to be capable of receiving this structure transiently or permanently. Among 

 these may be named horn, indurated jellies, tortoise shell, gums, resins, the 

 crystalline lenses of fishes or animals, &c., &c. 



When cylinders, tubes, rhombs, or other geometrical forms of well-annealed 

 glass are subjected to a sudden inci'ease of temperature acting upon all their sur- 

 face, as by immersing them in hot water or hot oil, there will be seen within 

 them, by polarized light, systems of symmetrical figures, circular and concentric 

 in cylinders, and dependent on the form of the solid for their shape in other cases, 

 bearing a striking resemblance to the rings seen in crystals. Like those rings, 

 these figures are marked by a cross, which changes from black to white with 



