Form of Saecharimeter, 19 



Let C A be the trace of the plane 

 of polarization of the right half of the 

 pencil, and B that of the other half. 

 Let C D bisect A C B. Then, if C E 

 represent the plane of polarization of 

 the light which alone the analyzer will 

 allow to pass, when the analyzer is 

 turned so that C E is perpendicular to 

 C A the right-hand side of the field is 

 dark. When C E is perpendicular to 

 C B the right-hand is partially illuminated (as C A has a compo- 

 nent along C E), while the left-hand is dark. Halfway between 

 these positions, when C E is perpendicular to C D, both sides 

 appear equally illuminated. The analyzer being turned round 

 till this equality of illumination is obtained, its position is 

 noted on the attached circle. When an active substance is 

 now inserted in the path of the rays, the planes C A, C B are 

 both rotated through the same angle, and the analyzer has to 

 be rotated through this angle to give the equal illumination 

 once more. The circle again being read, the difference of 

 readings gives the rotation due to the interposed substance. 



In Jellett's saecharimeter the inclination of the planes of 

 polarization of the two halves of the field is obtained by inter- 

 posing a prism of Iceland spar. This is formed by cutting a 

 rhomb nearly parallel to its optic axis, reversing one of the 

 pieces, and then cementing the two together again with the 

 plane of separation bisecting the pencil of rays. 



In Laurent's instrument, for which homogeneous light is 

 used, half the pencil is passed through a plate of quartz cut 

 with its axis in the surface and parallel to its edge, the thick- 

 ness being such that the extraordinary is retarded half a wave- 

 length behind the ordinary. On emergence the directions of 

 vibration in the two parts of the pencil, one of which has tra- 

 versed the quartz, are equally inclined to the edge of the 

 crystal. The inclination of the two to each other can be very 

 easily altered by simply turning the polarizer. 



The following arrangement is in place of the Iceland spar 

 in Jellett's instrument, and of the quartz plate in Laurent's. 

 It seems to be somewhat simpler, and gives fairly good results. 



A circular plate of quartz cut perpendicular to the axis is 

 divided along a diameter, and one half slightly reduced in 

 thickness. The two halves are then reunited and interposed 

 in the path of the pencil and at right angles to its direction. 

 Since one half of the pencil passes through a slightly greater 

 thickness of quartz, its plane of polarization is slightly more 

 rotated than that of the other half ; and the pencil therefore 



C2 



