CANE SUGAR. 



been turned the right way, and by trial and error the sharpest end point is 

 found ; after it has been determined the screw which was not used in the 

 adjustment is tightened until it just bears on the sleeve, after which the 

 second screw is turned home ; the scale adjustment to zero is then made. 



In other instruments the analyser is held in a brass tube which rotates 

 easily within an outer tube forming the casing of the instrument ; on the 

 casing is formed a slot covered by a collar ; two pins pass through the collar 

 and slot and screw into the tube which carries the analyser ; by loosening the 

 screws the analyser can be rotated about its longitudinal axis through 

 the length of the slot. 



Polarizer Adjustment. The instruments employing the Jellet 

 prism have the polarizer fixed in position ; the instruments employing the 

 Lippich polarizer are capable of adjustment in both polarizer and half prism ; 

 the former is capable of rotation by the last-mentioned device above. The half 

 prism is usually capable of two adjustments laterally by means of three screws 

 which pass through the casing ; two of these screws pass into the holder of 

 the half prism, and one bears on the holder ; rotation of the half prism is 

 effected by a system of screws and slots substantially as described above. 



In the Schmidt & Haensch triple shade instrument one of the half prisms 

 is capable of adjustment by means of coned screws bearing eccentrically on the 

 holder of a half prism ; adjustment is made by trial and error as described for 

 the analyser; this adjustment is necessary when the outer portions of the 

 triple field are unequally in shadow. 



Adjustment of the prisms is a delicate operation and should only be 

 undertaken when absolutely necessary, but no damage need be anticipated 

 when the theory of the instrument is understood and when care and patience 

 are used. If an old disused instrument is available, it is better first to acquiie 

 familiarity with the construction from manipulation of its parts. 



REFERENCES IN CHAPTER XXI. 



1. This chapter is based on Landholt's classical treatise: " The optical 



Rotation of Organic Substances." 



2. Ann. Chem. Phys., 1840, p. 401. 



3. Brit. Assoc. Report, 1860, p. 13. 



4. Bull. Soc. Chem., 1870, p. 140. 



5. Zeits. fur lustrum., 2, 167; 14, 326. 



6. Dingler's Poly. Jour., 1897, p. 608. 



7. Bull. Assoc., 19, p. 101. 



8. Ueber ein neues Polaristrobometer. Bern, 1865. 



9. /. S. J., 108. 



10. Zeit. fur Rub. Ind., 1898, p. 835. 



11. Zeits. fur Rub. Ind., 1891, 338. 



12. Jour. Roy. Agric. Com. Soc. Brit. Guiana, December, 189 o. 



446 



