THE POLAEISCOPE AS APPLIED TO SUGAR ANALYSIS. 



cups used to apply eye washes, and a sprung shank is soldered on to this cup, 

 forming a piece as in Fig. 2/j.S ; this is fixed on to the focussing telescope, the 

 size of the shank being such that it allows the telescope tube to rotate easily 

 within it. 



Variation in Optical Activity of Quartz. The optical 

 activity of quartz increases with the temperature, and is expressed, according 

 to Jobin, 10 by the equation a* = a (I + '000146), where a* is the specific 

 rotation at t, and a that at a lower temperature. Hence, if a quartz wedge 

 compensating polariscope, adjusted at the standard European temperature of 

 20 C., be used in the tropics at a temperature of 30 C., a less thickness 

 of quartz will be required to compensate for the rotation of a sugar solution. 

 This error is small but still appreciable. Suppose at 30 C. a reading of 96-0 

 be observed, then at 20 C. the reading would be 96 (1 + -000146 X 10) 

 = 96.14. 



Fm. 247. 



Similarly, a quartz control plate standardized at 20 C. will give the same 

 reading at 30 C. in a quartz wedge compensating instrument, but will give a 

 higher reading at the higher temperature in an instrument compensating by 

 rotation of the analyser. 



Polariscope Tubes. The older form of polariscope tube is shown 

 in Fig. 2!9. It consists of a glass or metal tube, the ends of which are ground 

 exactly flush. In filling a tube with a solution for observation a glass disc is 

 placed on one end and secured in position by screwing on a metal cap. The 

 tube is held vertical and the solution is poured in until it fills the tube, and 

 rises by capillarity over the upper surface ; a second glass disc is then slid on 

 and secured in position by a metal cap. 



441 



