CHAPTER XXIV 



THE POLARIMETER* 



THIS chapter treats of the principles involved in the determination of cane 

 sugar through its action on plane polarized light, the subject being carried 

 so far as to enable the operator to appreciate the principles of the methods 

 and the construction of the instruments that he employs. For more detailed 

 treatment reference should be made to the larger works of Landolt or of 

 Browne, and to textbooks on Physics and on Light. 



Nature of Light. Ordinary light is accepted as being the effect on the 

 eye of vibrations in the ether which take place in all directions. According 

 to the wave length of the vibrations, the eye receives the sensation of colour, 

 white light being the effect on the eye of the simultaneous receipt of ether 

 vibrations of different wave lengths and colours, which severally go to form -&/ 

 the colours of the spectrum, into which white light is split up on its passage 

 through a prism. 



Polarized Light. By means of certain devices the vibrations of ordinary 

 light may be confined to one plane and such light is called polarized light. 

 The position of the plane may be determined by many devices and hence, 

 if the plane be rotated, the angle through which rotation has occurred may 

 be measured. 



Rotation of the Plane of Polarization. There are certain bodies character- 

 ized by the possession of an asymmetric carbon atom (or atom which is at 

 least quadrivalent), which have the property of rotating the plane of polariza- 

 tion when a beam of such light is passed through them. Generally the 

 magnitude of the angle through which the plane is rotated is proportional 

 to the concentration of the active material and the length of column through 

 which the passage of light occurs. Hence if the rotation is known for one 

 definite length and concentration, the composition of an unknown solution 

 can be found when the length of column thereof and the rotation produced 

 thereby is known. 



Cane sugar is but one of very many bodies of the class mentioned above, 

 and, owing to its commercial importance, polarimeters are built specially 

 designed and graduated for sugar analysis. Such instruments are often 

 called saccharimeters, but it is to be understood that instruments designed 

 for general work can be used equally well for sugar analysis, and with some 

 types the converse is also true. 



- Xx/vvi 



* In English the word " polariscope " has come to mean an instrument devised to measure the rotation of ^ 



the plane of rotation. The appropriate use of the word is to designate an instrument or device to see or examine 

 the phenomena of polarized light, and in this sense the word is correctly employed. Polarimeter is used here when 

 reference is made to an instrument measuring rotation. 



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