58 



POLARISATION OF LIGHT. 



-slices A, B, which form the summit and 

 base of the prism to the thickness of 

 about the 1 00th of an inch, and examine it 

 either by the miscroscope or by polarised 

 light, we shall perceive no difference 

 between them and ordinary crystalline 

 plates, excepting that the colour of the 

 single system of rings which they pro- 

 duce are peculiar, as described in Chap. 

 IX. p. 34. A number of veins merely ap- 

 pear at the edges, as shown in the figure. 

 By removing other slices, which may be 

 easily done by a knife, as the mineral 

 splits with great facility, and exposing 

 them to polarised light, we shall observe 

 that they exhibit the beautiful tesselated 

 figure shown iny?g-. 71. The outer case, 



Fig. 71. 



M N O P, which, as it were, binds the 

 interior parts together, is composed of a 

 great number of parallel veins or plates 

 which, from their minuteness, display 

 the colours of striated surfaces. Within 

 this frame are contained no fewer than 

 nine separate crystals, viz. the central 

 lozenge abed, the four prisms A, B, C, 

 D with trapezial bases, and the four 

 triangular prisms e h I, I m n, and 

 gfc, all of which are separated from 

 one another by distinct lines or veins. 

 By means of the microscope we can 

 easily see the lines eh I, Imn, nkg, 

 and gfe. The central lozenge abed is 

 is seen much less frequently, and the 

 radial lines ha, c k,fd, b m can only be 

 recognized by a particular mode of 

 throwing the light upon the plates, 

 though they are easily seen by -polarised 

 light. 



But the most remarkable circumstance 

 in this composite structure is, that the 

 central lozenge abed has only ONE 

 axis of double refraction, while the four 

 prisms A, B, C, D, have TWO axes. In 

 A and D the planes of the two resultant 

 axes are coincident, and lie in the direc- 

 tion of the diagonal M N ; and, in like 



manner the planes of the resultant axes 

 of B and C lie in the other diagonal 

 O P. This combination of crystals with 

 one and two axes, may be very easily re- 

 cognised by holding the plate M O N P 

 in the position DEFG of ./?. 39, and 

 turning it about its axis. In every po- 

 sition of the plate the lozenge abed will 

 be dark, while the crystals A> B, C, D 

 will be luminous when the sides M O, 

 O N are parallel or perpendicular to the 

 plane of primitive polarisation. 



Beautiful as this structure is, it is yet 

 far surpassed by that of another variety 

 of Faroe apophyllite, the extraordinary 

 organisation of which is thus de- 

 scribed by Dr. Brewster. 



" Among the various forms in which 

 the apophyllite occurs, there is one from 

 Faroe of a very interesting nature. The 

 crystals have a greenish-white tinge, and 

 are aggregated together in masses. The 

 quadrangular prisms are in general below 

 one-twelfth of an inch in width ; they are 

 always unpolished on their terminal 

 planes ; they have the angles at the 

 summit more deeply truncated than the 

 other quadrangular prisms from Faroe ; 

 they are always perfectly transparent, 

 and may sometimes be detached in a 

 complete state, with both their terminal 

 summits. 



" In examining this variety of apophyl- 

 lite, I was enabled, by the perfection of 

 the crystals, to study their structure, 

 through the natural planes, and at right 

 angles to their axes. The phenomena 

 which this investigation presented to me 

 were of a very singular and unexpected 

 nature. In symmetry of form and 

 splendour of colouring, they far sur- 

 passed any of the optical arrangements 

 that I had seen, while they developed a 

 singular complexity of structure, and in- 

 dicated the existence of new laws of 

 mineral organisation. 



" When a complete crystal of this 

 variety of apophyllite is exposed to po- 

 larised light, with its axis inclined 45 

 to the plane of primitive polarisation, 

 and is subsequently examined with an 

 analysing prism, it exhibits, through 

 both its pair of parallel planes, the ap- 

 pearance shown in fig. 72. In turning 

 the crystal round the polarised ray, all 

 the tints vanish, re-appear, and reach 

 their maximum at the same time, so 

 that they are not the result of any he- 

 mitropism, but arise wholly from a sym- 

 metrical combination of elementary 

 crystals possessing different primitive 

 forms and different refractive and po- 



