Effect of Bodies on Light and Seat. 395 



talline nature of their construction. Among the organic substances 

 ' < may be named horn, indurated jellies, tortoise shell, gums, resins, 

 the crystalline lenses of fish or animals, &c." ( Barnard.) 



If a difference in the relative position of the molecules of a non-crys- 

 talline transparent body be brought about, as may be done by pressure, 

 or strain or tension, it becomes, for the time being, doubly refractive 

 and chromatic under polarized light. Thus, if a bar of glass be bent, 

 it is on a strain and the molecules of one side are stretched apart, while 

 the other side is compressed and the molecules crowded together. Light 

 passed through the bar edgewise will be decomposed, because the wave 

 lengths are unevenly shortened and therefore the emerging light will be 

 colored. The same is true when glass is subjected to pressure, or to 

 torsion, or to mechanical vibrations, or to uneven expansion by heat. 

 Grlass can be made chromatic permanently by being heated to the point 

 of fusion and then cooled so rapidly as to leave the parts on a strain. 

 Many articles of glassware are so imperfectly annealed after being 

 formed that they are on such permanent strain, as tubes, and lamp 

 chimneys, stems of wine glasses, stoppers of bottles, &c. 



Rotary Polarization. If a plate of Quartz be cut from the crystal 

 perpendicular to its axis, it possesses the power of twisting the plane 

 of vibration to a degree depending on the thickness of the plate. When 

 such a plate is placed between the polarizer and analyzer, and light 

 passed through them, there results a single color. If the analyzer be 

 turned in azimuth, say from left to right like a clock, the colors suc- 

 cessively change, ascending the chromatic scale as from orange to yellow, 

 yellow to green, &c. There are some of these quartz crystals, however, 

 in which the analyzer must be turned the other way, from right to left, 

 in order to cause the colors to ascend the scale. From this circumstance 

 these crystals have been named dextrogyre and Icevagyre, or right twisters 

 and left twisters. If, instead of a nicol prism, a piece of Iceland spar be 

 used for the analyzer in the experiment with the quartz plate both the 

 refracted rays will be transmitted, and then it will be seen that the or- 

 dinary ray is of a certain color and the extraordinary ray is of the com- 

 plementary color. If this analyzer be now turned the colors in each 

 ray will progressively change, but each will always remain the comple- 

 ment of the other. Observe that the crystals that produce this gyratory 

 progress of the light waves are cut across the optic axis and not parallel 

 with \\ so that we are to conceive the general direction of th'e light as 

 with the axis, but its movement as spiral. In amethyst the structure 

 appears to partake of both right and left gyration so that at the surface, 

 lines of neutral character are produced. The explanation of this rotary 

 polarization probably is, that the double refraction that takes place in 

 the crystals sends two spiral planes of light in the general direction of 



