INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 59 



two components, one vibrating parallel to OA and one to OB, and with 

 the corresponding refractive indices as in Fig. 21(b), and these refractive 

 indices can be calculated if n^, n^ and Q are known. Conversely, if the 

 refractive indices for sections making known angles to each other are 

 known (actually only two sections are necessary) then n^ and «„ can 

 be calculated as well as their orientation with respect to either section. 



The use of crossed Nicols. The major extinction position 

 A good deal of useful information can, however, be derived without 



actual measurement of refractive indices. The principles involved can 



perhaps best be seen in botanical material by considering the wall of a 



fibrous cell in longitudinal section, i.e. the wall BC in Fig. 28. Suppose 



a polarizing microscope is set up, in the usual way, with the direction 



of vibration of light in the lower Nicol prism (the polarizer, below the 



substage) and the upper one (the analyser, above the objective) at right 



angles to each other. Then the light coming up from the polarizer, 



vibrating exactly at right angles to that which the 



analyser will allow to pass, fails to penetrate to the 



eye-lens, and the microscope field is dark. If now 



the cell wall section we are considering is placed 



in the field of view then, in general, it will appear 



bright against the dark background. If the stage 



is rotated it will be found that the wall will 



darken completely four times per revolution. 



These four positions are exactly at right angles to 



each other; in two of them the edge of the wall is 



parallel to the direction of vibration of the polar- 

 izer, and in the others that in the analyser. The 



explanation is as follows: 



Looking down the microscope, let PP (Fig. 



29(a)) be the direction of vibration in the polarizer 



and AA that in the analyser, and let the wall BC 



be inserted with one edge making any angle d with 



the direction PP. Then the vibration along PP will 



have a component along both BC and DE (the n^ 



and «„ directions). Two vibrations will then pass representation^^of part 



upwards through the specimen. Since, however, of an elongated cell. 



the refractive indices of these are different, their gj^^ wall^'^as seen ^"in 



velocities are different (which is saying the same optical section. In re- 

 ^1 • cix TT • ^u r gion A the upper wall 



thmg, see p. 51). Hence smce the frequency, v, has been removed; in E 



of the light (the number of vibrations per second) both walls are present. 



