DOUBLE EEFEACTION. 115 



Iceland ,spar is favorable to observations upon double refraction, not only on 

 account of its wide separation of the n-fracted rays, but also because of the size 

 of the crystals which can be obtain 'd of this mineral, and of their beautiful 

 transparency. Its primitive crystalline form is the rhombohedron. Whatever 

 may be the configuration of the mass as obtained from its native bed, it will be 

 found to cleave with great ficility in directions paraUel to the fVices of the 

 original rhombohedron, and it is thus easily reduced to a form favorable for ex- 

 periment. The angles of the rhomboidal flices are 101° 55' and 78° 5'. Th« 

 inclinations of the faces upon each other are 105^ 5' and 

 74° 55'. Two of the solid angles are contained by three 

 of the obtuse angles of the rhomboids, and the other six 

 by two acute and one obtuse each. The diagonal con- 

 necting the two exceptional solid angles is the shortest 

 ^'S' ^- of the diagonals of the rhombohedron, and is called the 



crystallographic axis. These angles themselves are called the vertical, and the 

 other six the lateral, angles of the crystal. 



If a mark be made with ink upon a sheet of white paper — a small cross fa- 

 example — and a rhomboid of Iceland spar, two or three inches in thickness, be 

 laid over it, then in whatever position the eye may be placed above the upper 

 surface of the crystal, two crosses will be seen. If the crystal be turned about 

 upon its horizontal face, one of these images will remain motionless, and the 

 other will describe a circle around it. The motionless image will, moreover, 

 appear sensibly nearer to the eye than the other. If, mstead of a small mark, 

 wt take a straight line ruled entirely across the paper as an object, then, if the 

 eye be placed vertically over the line, and the crystal interposed, it will be seen 

 that th<^ nearer image is always a continuation of the part of \he line seen 

 beyond the crystal on each side, while the more distant one is more or less dis- 

 placed laterally. In revolving the crystal, moreover, this second image will 

 pass from one side to the other of the first, and .a position will be found (or 

 rather two positions, differing from each other by 180^) in which the two imajres 

 apparently coincide, though, as they are differently distant, they are merely 

 superposed. 



Until the discovery of this remarkable property in Iceland spar, refraction 

 was supposed to be governed in all cases by the law of Snellius. But it is im- 

 possibh that this should be true of both the rays in the present case. It is, in 

 fact, true only of that one which produces the nearer and fixed image, This is, 

 for distinction, called the ordinary ray ; tlie other, the cxtraxyrdinary. 



If the vertical angles of the rhombohedron be truncated perpendicularly to 

 the crystallographic axis, and the artificial faces thus formed polished, it will bo 

 found that when the crystal is laid over a small object upon one of these faces, 

 and the eye placed immediately over it, only one imi.ge will be visible. This is 

 not an illusion occasioned by the superposition of images differently distant ; 

 there is actually but one image. But if the emergent ray coming t(; the eye, by 

 which the object is seen, be at all inclined to the surtacc, tin- im)ge will b" 

 diipiicatcd, and the degree of separation of the two images will iucrcasi! witb 

 the inclination. If the lateral edges of the crystal are cut away, so as to i'orai 

 a parallelopipcdon, whose faces arc parallel to the crystallographic axis, and tin- 

 crystal be laid on its side, the separation of the images will be at its maximum. 

 In this case, if the emergent visual ray be perpcnulicular to the surface, the tw;- ^ 

 images will be superposed, but the duplicity will be very perceptible. 



It appeal's, then, that there is one direction in the crystal, in which light may 

 pass without double refraction, and tha., this direction corresponds with that of 

 the cry.-;tallograi)hic axis. This direction is also called the (ypiic axis; but the 

 term optic axis, it must be observed, is not intended to denote a particular Ibic, 

 but only a particular direction, and in the present case it is a line anywhere in 

 the crj'stal parallel to the axis of symmetry. 



