CANE SUGAR. 



Specific Rotatory Power is the rotation at 20 C., expressed in angular 

 degrees, of plane polarized light corresponding in wave length to that of the 

 yellow D line of the solar spectrum, when the light passes through a column 

 of a solution of the active material 10 cm. long, containing one grm. of active 

 material in one c.c. 



Kind of Light employed. For scientific purposes it is necessary 

 to refer all measurements to one source of light, and the light obtained from 

 an incandescent sodium salt corresponding to the D line of the solar spectrum 

 has been adopted. For technical sugar analysis polariscopes are adapted (in 

 nearly all cases) to use ordinary white light. In what follows in explaining 

 the use of polariscopes as apart from saccharimeters, it is assumed that the 

 source of light is homogeneous and monochromatic. 



For the theory of this source of light reference should be made to 

 Landholt's classical treatise. 



FIG. 228. 



The Nicol Prism and Means of obtaining Polarized 

 Light. Polarized light is obtained for use in the polariscope by means of a 

 nicol prism or some development of it. If ordinary light pass through 

 crystals of certain bodies, of which Iceland spar is an example, it suffers 

 double refraction, and is split up into two rays, one of which is known as the 

 ordinary and the other as the extraordinary ray. A nicol prism is formed out 

 of a prism of Iceland spar, the ends of which make angles of 71 and 109 ; 

 the ends of the crystal are ground until they form angles of 68 and 112; 

 the crystal is then split diagonally, and at right angles to the plane of the 

 ends and to the principal axis, i.e., along a c, Fig. 228 ; the surfaces are then 

 polished and united with a film of Canada balsam. 



Let ee represent a ray of light entering the prism ; this ray is doubly 

 refracted into two rays, one taking the direction e p, and the other the 

 direction e h. e p is the ordinary, and e h the extraordinary ray ; the ray e p 

 on meeting the film of Canada balsam a c is reflected in the direction p g, 

 and provided that the ends of the prism have been ground down to angles of 

 68 and 1 12, will pass out of the prism before reaching the surface a d, and is 

 totally lost if the sides of the prism are blackened. The extraordinary ray i& 

 less refracted, and emerges from the prism in the direction kk, and is now 

 plane polarized light. 



428 



