CRYSTALS OF QUARTZ ACROSS THE AXIS. 135 



These riugs make tlieir appearance at thicknesses much greater than tliose 

 which produce color in hxmina; parallel to the axis. 



In examining plates of quartz cut across the axis as above described, Mr. 

 Arago observed a peculiarity of a remarkable kind, which is scarcely found in 

 any other natural crystal. The centre of the field was not dark in any position 

 of the analyzer, but was deeply and uniformly colored with a tint which varied 

 as the analyzer was turned. When a bi-refringent prism Avas employed as an 

 analyzer, the two images seen Avere constantly complementary in color, and as 

 the analyzer was turned they ascended in tint, in the order of Newton's scale, 

 from red to violet. Mr. Biot, in subsequent experiments, discovered that in 

 some crystals the ascent of the tints in the scale is produced by a right-hand 

 rotation, (the ordinary direction of a screw,) and in others, by a left-hand rota- 

 tion. These classes of crystals have been distinguished by the names right- 

 handed and left-handed crystals, or dcxtrogyre and Imvogyre. Sir John Her- 

 schel, at a later period, made the remarkable observation that these optical 

 peculiarities of the crystals are associated with a geometrical or crystallographic 

 peculiarity. The tetrahedral angles where the prism and terminal pyramid 

 of the crystal meet, are sometimes replaced by planes which encroach more on 

 the neighboring planes of one side than on those of the other. The same 

 occasionally happens Avith the lateral edges of the crystal. These faces are 

 called j)lagilLedral. If, as the crystal is held in the hand horizontally, Avith the 

 pyramidal A^ertex toward the observer, the plagihedral foces lean to the right — 

 that is, if they encroach most upon the faces to the right of them — the crystal 

 Avill be found to be optically dextrogyre, and the analyzer Avill have to be turned 

 in the direction of the movements of the hands of a AA'atch, in order that the 

 tints may ascend. 



Sir David Brewster's observations on these 

 crystals led to the discoA^cry that, when the 

 ystal is not very thick the uniformly tinted field 

 confined to the centre, and is surrounded by 

 ^'4. , a system of rings resembling those seen in Ice- 



^1^: ' hind spar, but in which the cross is imperfect. 



f|f, "!io figure exhibits the appearance. He also 



l|; ^ 'ind in that remarkable species of colored 



\V ^/" (juartz called amethyst, veins of right-handed and 



,#- W%^ left-handed crystallization alternating Avith each 



\ other in many parallel layers, and producing at 



''>i^^m^:<m:^'-:^^^i'^--'^^~^-^^^''<s^M^ ''^''''^" ^^1^'^^ces of contact lines of neutral character. 



■■-mm"-^ '"■"^•is.-::^.TJss^ '' ^&^ la some specimens the layers were found to be 

 Fig- IS. so extremely thin as to neutralize the rotatory 



poAver of the whole crystal, and in these instances the ordinary system of rings 

 with a perfect cross makes its appearance. 



In all these observations upon crystals in the direction of their optic axes 

 tlie number of rings is greatly increased by the use of monochromatic light. 

 The intervals between the rings are also, in such light, intensely dark. In the 

 case of quartz crystals, monochromatic light presents appearances in the centre 

 very little different from those s(?en Avhen the crystal is not present — that is to 

 say, it exhibits, as the analyzer is turned, a succession of maxima and minima, 

 separated from each other in azimuth 90 ^ But the absolute azimuths of these 

 maxima and minima are no longer what they AA^^re before the introduction of 

 the crystal : in other words, the plane of polarization has been turned to the 

 right or to the left, according to the nature of the crystal, througli an angular 

 distance proportioned to the thickness of the crystal. The peculiar kind of 

 polarization produced by quartz has, on this account, been called rotalary 

 polarization. 



