THE POLARISCOPE AS APPLIED TO SUGAR ANALYSIS. 



thus. An ordinary nicol prism (Fig. 231} is bisected in the direction of a 

 plane passing through the shorter of the two diagonals ; the sectional faces are 

 cut down 2, and the two halves reunited. In this way is obtained a 

 double nicol prism with two principal sections at an angle of 5 to each other. 

 Now in Fig. 232 let a f and e d represent the positions of the planes of 

 polarization of an analyser and polarizer of ordinary construction, when in the 

 position of crossed nicols ; that is to say, 0/and e d are at right angles to each 

 other. Now if a prism as devised by Cornu be substituted for the polarizing 

 nicol, the plane of polarization will now be represented by the line c o b ; let 

 the analysing nicol be rotated so that its plane of polarization is a /, perpendi- 

 cular to o c ; then that half of the field of vision will be in total darkness, but 

 the other half will be illuminated. Similarly, the right hand half of the field 

 of view can be made totally dark, and when the plane of polarization of the 



analysing nicol is in the position 



a /, the whole field is equally 



illuminated, and this position 



forms the critical position. This 



position is one of great delicacy, 



and a small change from the 



position a f results in unequal 

 FJG - 23L illumination. If the analyser be 



rotated so that it comes to the position d e, both halves of the field are 

 strongly illuminated, and receive nearly a maximum of light, but on slightly 

 rotating the analyser there is no sharp change as occurs when the plane of 

 polarization of the analyser is in the position af. 



The Lippich Half Shadow Device. This device, which is 

 now used in the best instruments of German make, obtains its half shadows 

 by the interposition of a small Nicol prism between the polarizer and the 

 analyser, as shown in Fig. 238. The half nicol is so fixed that its edge, (7, 

 lies in the axial plane of the apparatus, and divides the field of vision into twa 

 halves. Let the principal sections of the two prisms make an angle, a, with 

 each other. Light passing through the large nicol, a, and through the open 

 part of the field, vibrates vertically to the principal section of that prism. Of 

 the rays that pass through that half of the field covered 

 by the half nicol, only those pass that vibrate vertically 

 to the principal section. A field of vision is thus 

 obtained made up into two halves in which the planes 

 of polarization are inclined at a small angle to each 

 other, and the effect is precisely as described when dealing with the other forms 

 of half shadow instruments. In this instrument the analysing nicol is not set 

 parallel to the polarizing nicol, but makes with the polarization direction of the 

 half nicol a larger angle than it does with that of the large nicol, so as to correct 

 for the absorption of light which occurs in the passage through the small nicol. 



431 



