240 PRACTICAL MICROSCOPY. 



cap, each kind of apparatus being shown in Fig. 209, 

 where the eye-piece analyser is shown at B, the other 

 form at A. 



Sometimes a simple rhomb of Iceland spar, called also a 

 " double image prism," is used as an analyser, but is not so 

 generally useful as the Nicol prism previously described, 

 though a series of interesting experiments may be made 

 with it. 



There are several other methods of polarising light : By 

 means of the bundle of glass polariser, the plate of black 

 glass, the tourmaline plate, and the sulphate of iodo- 

 quinine crystal ; but as all these can be seen in almost any 

 treatise on optics, it seems scarcely necessary to more than 

 mention them here. 



The tourmaline plate may, perhaps, receive notice; in 

 this crystal the optical axis coincides with the axis of the 

 prism, and for optical purposes a plate is cut from it in a 

 plane parallel to this axis. There are many objections to 

 the use of a tourmaline plate, the principal being that a 

 considerable thickness is required, as unless the ordinary 

 ray is completely absorbed, the emergent light will be only 

 partially polarised, and this requisite thickness produces a 

 very undesirable colour. 



When the analysing prism is placed immediately above 

 the objective it stops a considerable amount of light, 

 though it gives a full field, while when placed over the 

 eye-piece a smaller field but brighter image is obtained. 



It is often useful to produce certain colour effects when 

 working with polarised light, and with this end in view a 

 plate of selenite is interposed between the polariser and 

 the object. Films of varying thicknesses are used ; they 

 are mounted in various ways, as selenite stages, object- 

 carriers, and the like. Perhaps the best form of selenite 

 stage is that made by Mr. Swift; it is shown in Fig. 210; 



