THE POLARIMETER 109 



from its original position, when the whole light will again pass 

 through freely. 



Many liquids and solutions of solids possess what is known 

 as optical activity, which means that they can rotate the plane 

 of vibration of a ray of polarized light passing through them ; 

 so that, on emergence from the liquid, the new plane is in- 

 clined either to the right or to the left of the original plane. 



This is known as the rotation of the plane of polarized 

 light. 



Laurent's Half-Shadow Polarimeter. This apparatus con- 

 sists of a tube containing two Nicol's prisms, of which one is 

 fixed and is known as the polarizer, while the other can be 

 rotated and is called the analyser. A quartz plate which 

 covers half the field of vision is fixed just behind the polarizer. 



The liquid or solution to be examined is contained within 

 a glass tube with polished ends, and is placed in position 

 between the quartz plate and the analyser. The analyser is 

 fixed in a tube which can be rotated, the degree of rotation 

 being read from a divided circle. Leaving out of consideration 

 the quartz plate, the beam of polarized light passes through 

 the liquid and so becomes rotated ; it follows, therefore, that 

 the vibration plane of the analyser will no longer be at right 

 angles to the plane of polarization of the light striking it, 

 therefore light will enter the analyser, and in order to bring 

 about complete extinction, the analyser must be rotated either 

 to the right or to the left This angle of rotation is a measure 

 of the optical activity of the substance under observation, and 

 according to the direction of rotation, the substance is termed 

 dextro- or laevo-rotatory. In Laurent's polarimeter the illu- 

 mination is obtained from a sodium flame, and this light before 

 entering the tube containing the liquid must pass through the 

 plate of quartz. When the instrument is set in the zero 

 position, the whole field is equally illuminated, but on in- 

 troducing the liquid, one half of the field becomes the darker ; 

 equal illumination can be obtained by rotating the analyser. 

 If this position be passed, the field is once more unequally 

 illuminated, but in a reverse manner, that is to say, the half 

 which was originally dark is now light, and vice versa. 



As the exact position of equal illumination is somewhat 

 difficult to determine, several readings should be made and the 

 mean of these taken as the correct value. 



