INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 61 



to subsequently as the m.e.p.) and this in turn may be seen from Fig. 

 27 to correspond to the projection, in the plane of the section, of the 

 direction of the cellulose chains in the wall. To determine this latter 

 direction it remains therefore only to ascertain which of these two 

 extinction positions is the m.e.p. 



Newton's colour scale. The determination of the m.e.p. 



This can be done quite readily after noticing that, if white light is 

 used, the specimen in the bright position (the 45° position) between 

 crossed Nicols is coloured and considering the interpretation of this 

 fact. Let us notice first that, if monochromatic light of wavelength A 

 is used, then the wavelength in the wall will be less than this; suppose it 

 is Ay for light vibrating parallel to the m.e.p. and A„ perpendicular to 

 this (Ay<AJ. Then, if the wall thickness is d, the number of wavelengths 



in the wall thickness will be - and - respectively. There will therefore 



Ay Aq 



be a difference in the number of wavelengths ^^ ^(y "~ y ) ^°^ ^^ ^^ 



for this reason that the wall is bright. Multiplying this by A gives the 

 difference in the two paths in the same units as for d. 



"^ir-i) 



or, remembering the definition of refractive index (p. 51) 



p=iny—njd. 



This is called thtpath difference. If the path difference is zero, then the 

 substance is isotropic (like glass) and the object is dark in all positions. 

 As the path difference increases, the wall becomes bright in the 45° 

 position; but when the path diff"erence is equal to one wavelength then 

 the vibrations leaving the wall are in step again, just as they were on 

 entering; they therefore reform the same vibration PP as entered the 

 wall and this is extinguished by the analyser. The object is then dark at 

 all azimuths just as if the wall were isotropic, and the same thing is 

 obviously equally true if the path difference is 2A, 3A or any whole 

 number of wavelengths. 



If now white light is substituted for the monochromatic Hght, then 

 in general the object will appear coloured. For assume, for instance, 

 that the path difference is equal to the wavelength of green light; then 

 green light, and green light only, will be completely missing from the 

 light transmitted by the analyser and this light will then be white light 



