INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 69 



this corresponds to one complete vibration, i.e. to one path length of X. 

 During the same time the point P makes one complete revolution, i.e. 

 passes through 2n radians. Hence, 



a path difference of A=a phase difference of In. 



Therefore, 



a path difference of (w^— «Ji/=a phase difference of{n^—nJd . 27ijX=^. 



This is what is measured by the de Senarmont compensator. In use, the 

 object is placed at the 45° position as usual, and illuminated by sodium 

 light. A plate of mica, cut so that its path difference is exactly one 

 quarter of a wavelength is then inserted in the light path between the 

 object and the analyser, with its m.e.p. parallel to the direction of 

 vibration of the polarizer. Now if the analyser is turned it is found that 

 the brightness of the object first diminishes, passes through a minimum 

 (the intensity of the field and therefore approximately zero) and in- 

 creases again. The angle through which the analyser has to be turned to 

 make the intensity a minimum is numerically equal to one-half of the 

 phase difference required. It should be noted that since therefore a 

 rotation of 180° is equivalent to a path difference of one wavelength, 

 the method is a very sensitive one. Once the phase difference is known, 

 the birefringence can readily be calculated from the above equation. 



Dichroism 



Before leaving optical methods of structure determination, mention 

 must be made of a phenomenon associated with polarization in some 

 crystals since, although cellulose itself does not show this phenomenon, 

 it can be dyed with stains which do, and this can give useful information. 

 So far, attention has been confined to anisotropic bodies in which the 

 absorption of light in the body is the same for both directions of 

 vibration. Some crystals, however, show a greater absorption for some, 

 or for almost all, wavelengths in visible radiation for light vibrating 

 parallel to one or more of the axes of the index eUipsoid. Such crystals 

 are said to be pleochroic; here we shall deal with the simplest case in 

 which the absorption is greater in only one direction, a phenomenon 

 called, therefore, dichroism. Some of the stains used customarily to 

 stain cellulose show this phenomenon, notably iodine and congo red. 

 Here we shall confine our attention to iodine. If a cellulose wall is 

 purified, i.e. the incrusting substances such as lignin removed, and is 

 stained in iodine and 70 % sulphuric acid, the cellulose can be seen to 

 turn blue both in bulk to the naked eye and in single cells under the 

 microscope. Suppose a wall such as BC, Fig. 28, is stained and examined 



