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YIII— A POLAEISCOPE FOE DISSECTING AND 

 POCKET MICROSCOPES. 



By Peof. Ct. H. Bryan, F.K.S. 



iBead October 20, 1920.) 



Hitherto the polariscope has been usually regarded as an accessory 

 ouly of the compound microscope, for which purpose it generally 

 takes the form of a pair of Nicol prisms. There are, however, many 

 cases in which polarized light may be usefully employed in 

 connexion with a dissecting microscope or even a hand magnifier, 

 and in view of the scarcity of Iceland spar and the cost and small 

 size of the prisms obtainable, interest attaches to the well-known 

 alternative method of polarizing light by allowing it to fall on a 

 bundle of plates of glass or similar material placed at a suitably 

 oblique angle of incidence. 



An application which should appeal to every microscopist lies 

 in the fact that dust fibres, which in ordinary light are almost 

 invisible in Canada balsam during the process of mounting a slide, 

 are readily detected with a polariscope, and can be easily removed 

 under a dissecting microscope fitted with this accessory. With the 

 same apparatus I have been able to show the " black cross " effect 

 and the colours produced by selenite on the shells of the embryo 

 Docis while alive and moving in the water ; and, on the other hand, 

 the absence of polariscopic effect in the floats of Velella shows 

 these to be non-calcareous in structure and formed of material 

 which is not of a doubly refracting character. 



In some early treatises on natural philosophy a bundle of glass 

 plates is called a polarizing frwrne when the reflected light is used, 

 and a polarizing pile when use is made of the light transmitted 

 through the plates. As the light received in the former arrangement 

 is made up of a number of portions produced by reflection at the 

 several surfaces of the plates, a polarizing frame cannot be used as 

 an analyser, though it would be possible to employ a single reflect- 

 ing surface for the purpose. On the other hand, if the light is 

 incident at the polarizing angle on a pile of plates, the polarized 

 beam passes directly through the series of plates without any portion 

 of it being reflected, so that on looking through the pile a single 

 image only is seen, and it will be found that with an analyser of 

 this description the loss of definition is far less than might be 



