STARCH. 287 
under a quarter or eighth inch object glass, the cells of the cortex 
and medulla will be seen to be filled with rounded grains, most of 
which show traces of a division into three separate granules. In 
the balsam preparation it is not easy to see the striation for the 
reason above given, but some grains are sure to shew it under a 
suitable illumination. It will then be seen that the layers do not 
encircle the grain as a whole, but each granule has its own hilum 
surrounded by its own concentric layers. Very often one or two 
divisions are much more pronounced than the rest. 
Starch assumes a most characteristic appearance under polarised 
light. Space will not allow, nor is it needful to enter into an 
account of the nature of polarised light, with which we must 
assume our readers to be in some degree familiar. It will suffice 
here to draw attention to the phenomena presented by its use. 
When starch is examined under crossed Nicols the field remains 
dark, but each granule assumes a glistening grey appearance, as if 
self-luminous, and is marked with a black cross. If, then, the 
object be slowly rotated in the field of view, it will be seen that the 
cross remains fixed with regard to the field, one pair of its arms 
being parallel to the principal plane of the polariser and the other 
parallel to the principal plane of the analyser. As the arms of the 
cross, however, are frequently curved, their direction does not 
always appear to coincide with these planes. During the rotation, 
in fact, the grain appears to be turned round underneath the 
stationary cross. 
If, the object remaining stationary, the polariser or the analyser 
be rotated, the cross will be seen to rotate with it but with only half 
its angular velocity, so that to make a complete rotation of the cross 
the analyser or polariser must be rotated twice. If now a thin 
film of selenite be interposed between the polariser and the object 
while the nicols are crossed, and be rotated until it gives the 
brightest field, most beautiful chromatic effects will be obtained. 
The field will assume a colour dependent upon the thickness of 
the selenite film, and the interference crosses will be vividly 
coloured, the rest of the grains assuming a complementary colour. 
For instance if a yellow-blue selenite be employed and be so 
adjusted in the first instance as to give a blue field the crosses will 
be red at the edges merging into yellow in the centre, and the 
interspaces will be bright green. Then on rotating the analyser or 
polariser, as the blue field gradually merges into the complementary 
yellow, so the crosses rotate and change to their complementary 
colours. By means of the interference phenomena under polarised 
light, the compound nature of the grains in sarsaparilla is most 
clearly shewn, for each granule exhibits its own cross and its own 
chromatic effects. In studying this preparation, the medulla or 
pith is the most favourable part, for here the cells are largest and 
