154 



THE MICROSCOPE. 



the otliers. It presents a black cross, the arms of which 

 meet at the hilum. On rotating the analyser, the 



black cross disappears, and 

 at 90° is replaced by a white 



cross j another, but much 



Fig. 95. 



Potato Starch, seen under polarised 



Light- 



fainter black cross being per- 

 ceived between the arms of 

 the white cross. Hitherto, 

 however, no colour is percep- 

 tible. But if a thin plate of 

 selenite be interposed between 

 the starch-grains and the po- 

 iariser, most splendid and 

 delicate coloiirs appear. All 

 the colours change by revolving the analyser, and become 

 complementary at every quadrant of the circle. West and 

 East India arrow-root, sago, tapioca, and many other 

 starch-grains, present a similar appearance ; but in pro- 

 portion as the grains are smaller, so are their markings 

 and colourings less distinct. 



" The application of this modification of light to the 

 illumination of very minute structm'es has not yet been 

 fully carried out ; but still there is no test of differences 

 in density between any two or more parts of the same 

 substance that can at all approach it in delicacy. All 

 structures, therefore, belonging either to the animal, vege- 

 table, or mineral kingdom, in which the power of unequal 

 or double refraction is suspected to be present, are those 

 that should especially be investigated by polarized light. 

 Some of the most delicate of the elementary tissues of 

 animal, such as the tubes of nerves, the ultimate fibrill^ 

 of muscles, &c., are amongst the most striking subjects that 

 may be studied with advantage under this method of illu- 

 mination. Every structure that the microscopist is 

 investigating should be examined by this light, as well 

 as by that either transmitted or reflected. Objects 

 mounted in Canada balsam, that are far too delicate to 

 exhibit any structure under transmitted, will often be 

 well seen under polarised light ; its uses, therefore, are 

 manifold."^ 



(1) Quekett's Practical Treatise on the Use of the Microscope. 



