36 THE CHEMISTRY OF CATTLE FEEDING 



one which is united to four different groups of atoms in the mole- 

 cule. Many such substances exist in three isomeric forms one 

 dextro-rptatory, one laevo-rotatory, and one inactive. The last is 

 probably a compound of the other two. Mixtures, in equal pro- 

 portions, of the first and second are also inactive. The optical 

 activity of one neutralises that of the other. 



Optical activity is measured by means of a polariscope. This 

 instrument is made in several different forms, but they all consist 

 essentially of two Nichol's prisms placed at opposite ends of a 

 metal tube. One of the prisms, called the polariser, is fixed in 

 position and the light which passes through it is polarised. The 

 other, called the analyser, is movable, and when adjusted so that 

 the corresponding axes of the two prisms are parallel, the polarised 

 light is transmitted by it, and the field appears as a luminous disc. 

 When the prisms are crossed, i.e. adjusted so that the corresponding 

 axes are at right angles, no light is transmitted by the second 

 prism and the field is not illuminated. 



If, now, a tube containing a solution of an optically active 

 substance be introduced between the two prisms, so that the 

 polarised light passes through the solution, the field will be again 

 illuminated, though the prisms remain crossed, owing to the 

 rotation of the plane of the polarised light. The analysing prism 

 is now turned round till the field is again obscured, and the angle 

 through which it has been turned is measured on a divided circle 

 attached to the instrument. This angle is called the angle of 

 rotation. The arrangement of the prisms and tube containing 

 the solution is shown diagrammatically in Fig. 3, and the external 

 appearance of the instrument in Fig. 4. 



As it is difficult to determine exactly the points of maximum and 

 minimum illumination, a " half shadow " device has been introduced 

 into the more modern instruments. The appearance of the field 

 in this form of apparatus is shown in Fig. 5. If the prisms are 

 not properly adjusted, one-half of the disc appears darker than 

 the other, and the neutral point is found by turning the analyser 

 till the two halves are equally illuminated. The introduction of the 

 optically active substance again causes the appearance of the half 

 shadow, and the angle of rotation is determined by turning the 

 analyser till the shadow disappears. 



The angle of rotation a is proportional to the density of the 

 solution and the length of the column through which the polarised 

 light passes. For purposes of comparison it is usual to con- 

 sider the degree of rotation produced by a solution containing 

 one gram of the substance in each c.c. in a tube one decimetre 



