APPENDIX 191 



pendicularly to its axis {q in fig. 75), then through an arrangement called 

 a compensator (r in fig. 75), then through a second nicol called the analyser, 

 and lastly through a telescope (L in fig. 75). 



Fig. 74. — Soleil's saocharimeter. 



The compensator consists of two quartz prisms (RR, fig. 75) cut perpen- 

 dicularly to the axis, but of contrary rotation to the plate just in front of 

 them. These are wedge-shaped, and slide over each other, the sharp end of 

 one being over the blunt end of the other. By a screw the wedges may be 

 moved from each other, and this diminishes the thickness of quartz inter- 

 posed ; if moved towards each other the amount of quartz interposed is 

 increased. 



The effect of the quartz plate {d, g) next to the polariser (i in fig. 75) 

 is to give the polarised light a -sdolet tint when the two nicols are parallel to 

 each other. But if the nicols are not parallel, or if the plane of the polarised 

 hght has been rotated by a solution in the tube, one half the field will change 

 in colotu" to the red end, the other to the violet end of the spectrum, because 

 the two halves of the quartz act in the opposite way. 



^}g-.-|f . — 1||_ 



'0 



^ V 



Fig. 75. — Diagram of optical arrangements in Soleil's saccharimeter. 



The instrument is first adjusted vnih. the compensator at zero, and the 

 nicols parallel, so that the whole field is of one colour. The tube containing 

 the solution is then interposed ; and if the solution is optically inactive the 

 field is still uniformly violet. But if the solution is dextrorotatory- the two 

 halves will have different tints ; a certain thickness of the compensating 

 quartz plate which is laevorotatory must be interposed to make the tint of 

 the two halves of the field equal again ; the thickness so interposed can be 

 read off in amoxmts corresponding to degrees of a circle by means of a vernier 

 and scale (E in fig. 75) worked by the screw which moves the compensator. 

 If the solution is laevorotatory, the screw must be turned in the opposite 

 direction. 



