2i8 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



AD 1 will form the resultant plane AR. The angle CAO = angle 

 CAR. 



R C O 



A 



Two beams of polarised light at an angle to one another will there- 

 fore reach the analyser. If the analyser be set parallel to the beam 

 AO arriving from the uncovered portion, this half of the field will appear 

 light and the other half will appear dark. If it be set parallel to the 

 beam AR coming from the covered portion, this half of the field will 

 appear light and the other half dark. By adjusting the analyser a 

 position will be found where the two halves will appear equally illu- 

 minated. This position is the zero point. 



The two halves of the field are illuminated by component portions 

 of the two beams. At the zero point the two prisms are almost in a 

 crossed position. The instrument is most sensitive under these con- 

 ditions, but the amount of light is at a minimum. 



In other polarimeters, such as Lippich's, a prism which has the 

 same effect as a quartz plate is placed in the centre of the field. The 

 centre and sides of the field appear dark or light. 



In determinations with such polarimeters monochromatic light 

 must be used ; for convenience, sodium light is generally used, and in 

 this case a cell containing potassium bichromate is introduced in front 

 of the polariser to cut off blue rays ; green light from a mercury lamp 

 is sometimes used. 



A polarimeter (Figs. 42, 43) will thus consist of a bichromate cell, a 

 polarising prism, a quartz plate over half the field, a trough to take the 

 solution to be examined, an analysing prism mounted in a movable 



FIG. 42. 



L = source of light A = lens to render rays of light parallel. B = polarising prism. 



C, C' = quartz plate. O = observation tube. D = analysing prism. E, F = telescope. 



(From Findlay's " Practical Physical Chemistry ".) 



circle graduated in degrees. There is, in addition, a telescope to focus the 

 edge of the quartz plate and a double vernier on each side of the circle 

 in which the analyser is mounted. This vernier is fixed and graduated 

 in fractions of a degree, or in minutes. 



