POLARISED LIGHT AND THEl MICROSCOPE. 129 



two rays are polarised the optic axes of the section. We notic 

 that evei-y section of a doubly refracting substance has two optic axes, 

 a?id these are perpetidicular to each other. 



If the section is not very thick, the beams of light will not 

 have separated when they emerge, and they will re-combine to 

 form a beam of unpolarised light. Thus, in PL VIL, Fig. i, the 

 extraordinary ray from A and the ordinary ray from B unite to 

 form an unpolarised ray, which emerges at b, and so on ; only the 

 extreme rays which emerge at a and e are polarised. If then, a 

 crystal of Iceland spar is laid over a piece of black paper in which 

 a sufficiently large square hole is cut, we shall see the appearance 

 shown in Fig. 2. In the central portion the two images of the 

 hole will overlap and the light will be unpolarised, while in the 

 portions not common to the two images the rays will be polarised 

 in different directions, as shown by the directions of the shading. 

 When the images are examined with an analyser which is slowly 

 rotated, first one image and then the other will be extinguished ; 

 but the central part will never be extinguished, so that the light 

 which emerges from this part is unpolarised. 



Hence, if we exa?nine a doubly refracting section with unpolar- 

 ised light, we shall not obtain any polariscopic appeara?ices. In 

 other words, an analyser without a polariser would not be suffici- 

 ent to form a polariscope. 



Neither shall we obtaifi any polariscopic appearances if we 

 examine the section with light polarised alojig either optic axis of 

 the section. For if the incident light be polarised, say, along the 

 optic axis of the extraordinary ray, it will be entirely refracted as 

 an extraordinary ray, and will emerge polarised ks it entered. If 

 the polariser and analyser are "crossed," we shall still get darkness, 

 and if the polariser and analyser are parallel we shall still get 

 light just as if no object were interposed. 



We are now left with only one alternative, namely — To obtain 

 polariscopic appearafices, we must exa?nine the section with light 

 polarised in a directio?i more or less diagonal to the optical axes 

 of the sectiofi. 



Before proceeding further, the microscopist should verify this 



by the following experiment with the polariscope and selenite, even 



if he does not procure a rhomb of Iceland spar for the purpose of 



carrying out the experiments above described. 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. III. k 



