CH. V.] POLARIMETER. 143 



the volume to the mark with cold distilled water, rinsing the boiling 

 flask out with small amounts of water. Mix carefully, and estimate 

 the invert sugar by Benedict's method. 



Calculation of results. 25 cc. of Benedict's solution = 0-0475 

 gram, hydrolysed cane sugar. The concentration found must be 

 multiplied by 2, owing to the dilution made in preparing the 

 hydrolysed solution. 



E. The theory and use of the Polarimeter. 



Waves of ordinary light vibrate simultaneously in all 

 directions perpendicular to its direction of propagation. 

 By means of certain contrivances it is possible to affect the 

 light so that the vibrations proceed in a single plane. Such 

 light is plane polarized. The plane in which the light waves 

 vibrate is called the plane of polarization. This conversion 

 of ordinary light into polarized light is generally brought 

 about by means of a modified prism of Iceland spar known 

 as a Nicol's prism. If a beam of light (fig. 19) falls on the 



Fig. 19. Crystal of Calc Spar. 



face of a rhombohedron of Iceland spar it divides on enter- 

 ing into two rays, unequally bent, both of which are 

 polarized, their planes of polarization being at right angles 

 to one another. 



The extraordinary ray (E) is the lesser refracted ray : 

 the ordinary ray (O) is the more refracted ray. Before the 

 calc spar can be utilised for polariscopic purposes one of the 

 rays must be eliminated. This is best effected by Nicol's 

 method of splitting down a crystal in a certain plane, grind- 

 ing down the natural ends to reduce the acute angles from 

 71 to 68, and uniting the faces by Canada balsam (fig. 20). 



