226 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



which occur not in a plane, but are curved. The two rays are circularly 

 polarised in opposite directions, one describing circles to the left, the other 

 to the right ; these unite on issuing from the quartz plate ; and the net 

 result is a plane polarised ray with the plane rotated to right or left 

 according as the right circularly polarised ray or the left proceeded through 

 the quartz with the greater velocity. There are two kinds of quartz, one 

 which rotates the plane to the right (dextro-rotatory), the other to the left 

 (laevo-rotatory). 



Gordon explains this by the following mechanical illustration. Ordinary 

 light may be represented by a wheel travelling in the direction of its axle, 

 and the vibrations composing it executed along any or all of its spokes (a). 

 If the vibrations all take place in the same direction, i.e. along one spoke, 

 and the spoke opposite to it (6), the light is said to be plane polarised. The 

 two spokes as they travel along in the direction of the arrow will trace out 

 a plane (see fig. 73) between b and b'. If this polarised beam be made to 



travel now through a solution of sugar, the net result is that the plane so 

 traced out is twisted or rotated ; the two spokes, as in bb', do not trace out 

 a plane, but we must consider that they rotate as they travel along, as 

 though guided by a spiral or screw thread cut on the axis, so that after a 

 certain distance the vibrations take place as in b" ; later in &'", and so on. 

 This effect on polarised light is due to the molecules in solution, and the 

 amount of rotation will depend on the strength of the solution, and on the 

 length of the column of the solution through which the light passes, or, in 

 the case of a quartz plate, on its thickness. 



If a plate of quartz be interposed between two nicols, the light will not be 

 extinguished in any position of the prisms, but will pass through various 

 colours as rotation is continued. The rotation produced for different kinds 

 of light being different, white light is split into its various constituent colours ; 

 and the angle of rotation that causes each colour to disappear is constant for 

 a given thickness of quartz plate, being least for the red and greatest for the 

 violet. These facts are made use of in the construction of polarimeters. 

 Polarimeters are instruments for determining the strength of solutions of 

 sugar, albumin, &c., by the direction and amount of rotation they produce 

 on the plane of polarised light. They are often called saccharimeters, as 

 they are specially useful in the estimation of sugar. 



POLARIMETERS 



Soleil's Saccbarimeter. 'This instrument (see fig. 74) consists of a Nicol's 

 prism, d, called the polariser : this polarises the light entering it, and the 

 polarised beam then passes through a quartz plate (6 in fig. 74), 3*75 mm. 



