482 CHAPTER XXIV 



rotation, equal in magnitude and opposite in sign to that due to the active 

 material, introduced, so that the critical position again appears. It consists, 

 Fig. 302, of a plate of levo-rotatory quartz, c, and of two wedges of dextro- 

 rotatory quartz, a and b. By means of a rack and pinion gear, one of the 

 wedges is capable of being slid past the other, so that the combined thickness 

 of the system is capable of being varied. On the moving wedge is fixed a 

 scale graduated in single degrees from 30 to 105°, and on the fixed plate of 

 quartz is mounted a vernier. When the scale is at zero, the combined 

 thickness of the dextro-rotatory wedges a and b is equal to that of the 

 levo-rotatory plate c, so that the effect of the system is zero. By sliding 

 the scale towards the loo-point, a diminished thickness of dextro-rotatory 

 quartz is introduced, so that the effect of the system is levo-rotatorv, and, in 

 instruments designed for sugar analysis, at the loo-point exactly neutralizes 

 the rotation produced by the normal weight of sugar dissolved in 100 c.c, and 

 observed in a 20 cm. tube*. 



The double compensator is a development of this device. In this arrange- 

 ment two sliding wedge compensators are fitted to the instrument. The milled 

 head controlling one is coloured black, and the other red. The observation 

 is made in the usual way with the black compensator. The active solution 

 under analysis is then removed, and neutrality obtained by adjusting the 

 red compensator. The readings on the red and black scales should be 

 identical, practically demonstrating the accuracy of the reading, for it is 

 very unlikely that an identical error should be made twice running, or that 

 both compensators should possess the same error in construction. The 

 arrangement of the wedges is shown in Fig. 303. 



Saccharimeters are usually only provided with a scale reading to — 30° ; 

 with this device, by setting the zero of the red scale to the right, negative 

 readings of any value can be obtained. 



In instruments with single wedge compensation, negative readings of 

 magnitude greater than the scale permits may be obtained by inserting in the 

 path of the light a known positive rotation conveniently afforded by a 

 quartz plate. 



It is evident that the validity of this appliance depends on the nearly 

 equal rotation dispersion of quartz and of cane sugar. 



Equalization of a Fixed Rotation. — Between the polarizer and the analyser 

 permanently set in the critical position is introduced a fixed and known 

 rotation of sign opposite to that of the material being determined. The 

 material being examined is contained in a graduated and telescopic tube 

 the length of which is varied until balance is obtained. The observed 

 length of tube gives data to calculate the rotation of the unknown material 

 reduced to unit concentration and to standard length of tube. This device 

 was used by Jellett^^ and by Trannin^^. 



Optical Arrangements of Saccharimeters. — The arrangements of these 

 instruments from the time of Biot onwards are illustrated in Figs. 304-315. 

 The systems of lenses are not shown so as to avoid confusion. 



* It is somewhat confusing to appreciate the function of the analyser in instruments using the quartz wedge 

 system of compensation. In these instruments its function is to obtain a critical position acting in combination 

 with the polarizer. When monochromatic light is used it not only does this but serves as a means of measuriilg 

 the rotation. Conversely, quartz wedge compensation could be used, if so desired, with monochromatic light. 



