INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 63 



The use of this colour scale now offers a method of determining the 

 extinction positions much more accurately than can be done otherwise, 

 and, what is more important, of distinguishing between the major and 

 the minor refractive indices. For reasons which will become obvious, 

 the method is to interpose somewhere in the optical path of the micro- 

 scope, usually between the analyser and the objective, a slice of a crystal, 

 commonly selenite, whose path difference is such as to show a red 

 colour of the first order (Red 1). This is placed at 45° to the direction 

 of vibration of the polarizer (Fig. 30(a)) with the m.e.p. (w^^), say, 

 lying from the upper right down to the lower left, and the field now 

 appears red. The cell wall may now be placed on the stage and rotated. 

 When the m.e.p.s of the wall and plate are parallel, then the path 

 difference of (plate plus wall) is greater than that of the plate alone and 

 the colour of the combination is higher in Newton's series than is that 

 of the plate alone. The effect is that the wall appears blue or green, 

 depending on its path difference, against a red background (Fig. 30(b)). 

 If the two are at right angles to each other (Fig. 30(c)) then the colour 

 is equally low in Newton's Series and the wall appears orange or yellow 

 against the red background. When, however, one extinction position 

 of the wall is parallel to PP, then the wall has no effect on the Ught and 

 appears therefore the same colour as the field. Inspection of Table II 

 and Plate III, Fig. 1 will show that the change of colour with path 

 difference is particularly rapid in the neighbourhood of Red 1 and that 

 is why this plate is used. The method of determining the m.e.p. should 

 now be obvious. The stage is turned until the wall is the same colour as 

 the field, and the angular position of the stage is noted. This is repeated 

 several times and the average position obtained.* Now the stage is 

 turned slightly clockwise. If the colour of the wall changes to blue or 

 green (i.e. ascends Newton's Series, or shows an addition colour) then 

 the extinction position which is parallel to PP is the m.e.p.: if the colour 

 descends the series (shows a subtraction colour) then the position is the 

 minor extinction position. Assuming the wall to be in the former 

 position, then the analyser can be thrown out of the system and the 

 stage turned until the edge of the wall is parallel to the cross wire in the 

 eyepiece which in turn is parallel to PP. The angle through which the 

 stage must be rotated then gives the angle between the m.e.p. and the 

 cell length. 



A beautiful and instructive example of the colour effects associated 

 with structure is provided by starch grains, and a colour plate of starch 



* In practice the stage is then turned through 90° and the other m.e.p. obtained. 

 The resulting figure should be 90° removed from the first. This procedure ensures 

 correct colour matching. 



