26 



L. B. Tuckcrman 



the sign of the angle (1,1') changes the sign of k]. In measure- 

 ments on magnetic or electrical rotation, it mav also be eliminated 



j i I i i 1 — i—J. 



*//*»/ Adjustment _ (Xl'.O) J 



Fig. 2 



by reversing the rotation. If the effects are measured along the 

 lines of force, the rotation is reversed by reversing the field. If 

 the effects are measured at right angles to the lines of force the 

 rotation can only be reversed by rotating either the field or the 

 optical system through 90 . 



Type B. 



Bravais Biplate. 1 



Brace Elliptic Halfshade.-' 



Laurent Saccharimeter Halfshade. 3 



The Bravais Biplate consists of two halves of a doubly refract- 



1 Bravais, A. Ann. de Chim. et de Phys. (3), vol. 43, pp. 131-35, 1855. 



"Brace, D. B. Phys. Rev., vol. 18, pp. 70-88, 1904; Phys. Rev., vol. 19, 

 pp. 218-30, 1904. 



"Laurent, L. Dingier fount., vol. 223, pp. 608-14, 1877; C. R., vol. 89, 

 pp. 665-67, 1879. 



182 



