190 DR BREWSTER on a New Species of Double Refraction, 



T retires, the tints rise in the scale, and vice versa. When the 

 light is transmitted obliquely, the lines c d, &c. disappear. 



When a slice is cut off a summit with four planes, corres- 

 ponding to the edges of the cube, as in Fig. 7., the lines of no 

 polarisation a d, c d, bd, bf, ef, gf, are visible. The lines 

 a d, cd, and ef, gf, become sharper and narrower the more 

 the incident ray approaches to parallelism with the diameters 

 passing through d and/ When S s or df are in the plane of 

 primitive polarisation, the tints all vanish, because one of the 

 axes is then in that plane. When S s is inclined, so that S re- 

 cedes from the eye, the tints in S a b e rise, and those in sdbf 

 fall, and vice versa. 



In order to determine the character of the tints, we have 

 only to cross them with the axis of a plate of any crystal the 

 character of whose action is determined. When the polarised 

 tints shewn in Fig. 3. are crossed by a plate of sulphate of lime, 

 having its axis inclined 45 to the arms of the black cross AB, 

 CD, the tints all descend in the scale, and consequently the po- 

 larising action of the crystal is negative in relation to each of the 

 four axes of the icositetrahedron. In like manner, when the axis 

 of the sulphate of lime crosses any of the three sectors in di- 

 rections passing through d, Fig. 4., the tints in the sector thus 

 crossed descend in the scale. When the axis of the sulphate of 

 lime is placed in the direction S s, Fig. 7., the tints likewise 

 fall. 



In all these different directions, the tints polarised by the 

 crystal are exactly those of NEWTON'S scale, and have all the 

 properties of the tints of moveable polarisation. 



From an attentive consideration of the preceding experiments, 

 it is obvious that the phenomena of the tints exhibited in any 

 individual sector COB, Fig. 8., have no relation to the axis of the 

 icositetrahedron passing through O, considered as an axis of 

 double refraction. The axis of polarisation of every portion in 



