48o 



CHAPTER XXIV 



analyser, as shown in Fig. 296. ^^ The half nicol is so fixed that its edge, c, 

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

 halves. Let the principal sections of the two prisms make an angle 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 polariz- 



FlG. 296 



Fig. 297 



ation 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 absorp- 

 tion of light which occurs in the passage through the small nicol. As in the 

 Laurent instrument, a tripartite field can be obtained by the use of a second 

 half nicol, the appearance of one position of an unbalanced field being shown 

 in Fig. 297. 



Interference Devices. — In the passage of plane polarized light through 

 certain optical combinations, well-defined visual phenomena due to the 

 interference of light result. These phenomena in combination with polarizer 

 and analyser may be made to fix the position of the plane of polarization. 

 The Savart polariscope^® consists of two sections of calc spar, each 3 mm. 



Fig. 2 



Fig, 299 



thick, and cut at an angle of 45 degrees to the optic axis of the crystal. 

 The sections are finall}' cemented together, so that the principal sections 

 cross at right angles. If this device be arranged between parallel nicols 

 a number of horizontal bands or interference fringes occupy the field, as 

 shown in Fig. 298. When the principal section of the analyser forms an 

 angle of 45 degrees with the crossed sections of the Savart plate, and when 

 the principal plane of the polarizer is parallel with one of the crossed planes, 

 the field of vision is as in Fig. 299, and this is taken as the critical position. 



