THE MICROSCOPE. 



72. If either the reflecting surface or the crystal, placed under 

 the necessary conditions, be carried round a polarised ray, A' B' c' D', 

 so as to be successively presented to all sides of it, the ray will 

 be completely reflected or transmitted when it is presented to a', 

 the middle of the side A' B'. As it is moved from a' towards >', 

 the quantity of light reflected or transmitted will be less and less, 

 until it comes to b', when none will be reflected or transmitted, the 

 ray being wholly extinguished. As it is moved from b' to c', 

 the light reflected or transmitted, small in quantity at first, will 

 be continually greater and greater until it comes to c' the middle 

 of c' D', when the ray will be wholly reflected or transmitted. 

 As it is moved from c' towards d', the quantity of light reflected or 

 transmitted is less and less, until arriving at d' the ray is alto- 

 gether extinguished. After passing from d' towards ', the light 

 reflected, at first small, is more and more in quantity until it 

 comes in fine to a', when the ray is, as at first, wholly reflected or 

 transmitted. 



73. An instrument adapted to show the effects of polarised light 

 upon bodies on which it is incident or through which it is trans- 

 mitted, is called a POLARISCOPE, fig. 35, p. 65, and a polarising 

 microscope or MICRO-POLAKISCOPE, is a microscope by which the 

 observer is enabled to project polarised light upon the objects, and 

 to observe its effects when transmitted or reflected by them. 



Micro-polariscopes have been constructed in various forms, 

 some depending on polarisation by reflection, and some on polari- 

 sation by transmission. 



One of the most simple and most generally useful, consists of 

 two prisms of Iceland-spar, one of which, P, is placed under the 

 stage, so that the light by which the object is illuminated must 

 previously pass through it, and the other P' is placed in the body 

 of the instrument between the object-glass and the eye-glass, so 

 that before producing the image, the rays must pass through it. 



The light proceeding from P, and projected upon the object, 

 being polarised, and received, after passing through the object- 

 glass, by P', will be wholly or partially transmitted, or altogether 

 extinguished, according to the sides or poles of the ray to which 

 certain faces of the prism are presented. If, therefore, the instru- 

 ment be so mounted that the prism P' can be turned round its 

 axis, its faces can be presented successively to all sides of the 

 rays, so that the light will be in a certain position wholly trans- 

 mitted, and the image will be seen strongly illuminated. When 

 the prism is gradually turned round, the light transmitted will 

 be less and less, until the prism has been turned through a 

 quarter of a revolution, when the light will be wholly 

 extinguished, and the image will disappear. Continuing to turn 

 63 



