THE POLARISCOPE AS APPLIED TO SUGAR ANALYSIS. 



running, or that both compensators should possess the same error in 

 construction. The arrangement of the wedges is shown in Fig. 25^.. 



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, minus 

 readings of any value can be obtained. 



Use of Light Filter with Quartz Wedge Compensa- 

 tion. The application of the quartz wedge to compensate for the rotation of 

 sugar solutions is only possible because of the near coincidence of the rotation 

 dispersion of cane sugar and of quartz ; actually, the violet rays are a source 

 of error, and these may be shut off by passing the light through a crystal, or 

 through a dilute solution, of potassium bichromate. With dilute solutions the 

 effect of the violet rays is barely appreciable, but they become noticeable 

 (destroying the sensitiveness of the end point) at readings near the 100 point. 

 Some instruments are provided with a cell to be placed between the lamp and 

 the polarizer, which is filled with a dilute solution of potassium bichromate, 

 and in other patterns a plate of bichromate is inserted in the ocular of the 

 focussing telescope. This light filter should always be used when the incan- 

 descent gas or acetylene light is used. 



The disturbing effect of the violet rays is especially 

 noticeable when solutions of glucose are being observed. 



Adjustment of the Polariscope. Zero 

 Adjustment. After having obtained the end position 



with no active material in the body of the instrument. 

 FIG. 254. 



the scale should read zero. If another reading is obtained, 



the scale can be adjusted to zero by rotating a square headed pin located in 

 the Schmidt & Haensch and Peter's instruments at the side of the dust proof 

 casing of the compensator ; in the Fric instruments this pin is placed on the 

 top of the dust proof casing. A key fitting the pin is supplied with the 

 instrument. 



Analyser Adjustment. In case a sharp end point cannot be found the 

 analyser must be adjusted. In the earlier instruments, underneath the barrel 

 containing the analyser, will be seen a pin ; this pin carries a worm gearing 

 with a wheel cut on the sleeve containing the analyser ; rotation of this pin 

 by means of the key mentioned above controls the position of the analyser, the 

 best position of which must be found by trial and error. 



In other instruments, on each side of the tube containing the analyser, 

 will be found two screws with projecting heads. These screws have conical 

 points bearing eccentrically on the edges of two holes bored in the 

 rotating sleeve carrying the analyser. To adjust, both screws are loosened 

 and one screw is slightly turned, the end point is then found by altering the 

 position of the quartz wedges. If a sharper end point is found the screw has 



445 



