CH. V] POLARISCOPE. 109 



i.e. doubly refractive, object such as almost any vegetable 

 tissue, and it will be seen that parts of each of its 

 elements will appear black and parts white. The object 

 thus seen is said to be viewed "with crossed Nicols" 

 and the appearance is due to the arrangement of the 

 planes of polarisation in the object so altering the polari- 

 sation of the light that part of it and part only is capable 

 of passing through the analyser in its present position. 

 Hence the alternating light and dark markings. On shift- 

 ing the analyser through a right angle these markings 

 are now seen reversed in their optical properties. 



The arrangement of the markings is constant with 

 given objects. Starch grains exhibit on a black field a 

 large black and white Maltese cross with its centre at 

 the hilum of the grain. To see this well starch grains of 

 potato should be mounted in Canada balsam so as to 

 be quite transparent and when examined in ordinary 

 light quite invisible. Advantage may be taken of this 

 method for detecting anisotropic bodies otherwise small 

 and difficult to perceive. Thus the very minute crystals 

 of calcium oxalate occurring in many leaves are often 

 difficult to make out but if the leaf be decolorised and 

 rendered quite transparent by soaking it in strong 

 chloral-hydrate solution the distribution of these crystals 

 may be easily observed by their light crossed markings on 

 a black field. This is a valuable method for tracing the 

 seat and causes of the formation of calcium oxalate in the 

 plants 



1 Sehimper, Bot. Zeitung, 1888, p. 81. 



