INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 51 



0-5 mm. to a size comparable with that of the specimen. This is neces- 

 sary, not only for the need to mount the specimen in the beam with no 

 foreign body also in the beam, but also because however carefully the 

 slit is designed, there is always some scatter from the edges of the slit 

 and this must be kept well below the scatter from the specimen. This 

 alone, therefore, may increase exposure time by a factor of 10 x or 

 more when working with single cells. Again, the amount of reflecting 

 material is reduced perhaps by a factor of 800 x . This enormous 

 increase in exposure time can to some extent be offset by reducing the 

 specimen-film distance say to 2 mm., which would decrease the exposure 

 necessary by a factor of 900/4 (since beam intensity varies inversely as 

 the square of the distance) and by other methods, but nevertheless the 

 size of the specimen cannot be reduced indefinitely without a very 

 serious increase in exposure time. In point of fact, however, X-ray 

 diagrams have been pubUshed of single cells (20) and of even smaller 

 botanical specimens (21), even under conditions in which the exposure 

 time necessary was of the order of 150 hours. For some of the details 

 of structure on plant cell walls which it is very necessary to investigate, 

 however, it is as yet impossible to employ an X-ray technique and 

 recourse must then be made to optical methods. It is largely for this 

 reason that the polarizing microscope is of such importance in botanical 

 investigations. At the same time it must be stressed that this is by no 

 means the only reason why this instrument provides such a useful tool; 

 it can and does give information which could be obtained in no other 

 way. 



Polarized light and structural asymmetry 



The interaction of matter and light is usually expressed by the 

 refractive index, which is commonly interpreted in terms of a bending of 

 the rays of light when passing obliquely from one medium to another. 

 If the rays impinge, for instance, on the surface of separation of a block 

 of glass in air at an angle i to the normal, and the refracted rays make an 

 angle, in the glass, of r to this normal, then the refractive index is 

 expressed as sin //sin r. It is more instructive, and more apposite to the 

 needs of the following discussion, to define refractive index in another 

 way. When light passes from a less dense medium (say air) to a more 

 dense (say glass) then its velocity is diminished, and the ratio of the 

 velocities (air /glass) is numerically equal to the refractive index of the 

 glass. As already noted (p. 36), such a ray of light consists essentially 

 of a series of vibrations at right angles to the direction of propagation 

 and, following the electromagnetic theory of light, it is known that these 



