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ESSENTIALS OF CHEMICAL PHYSIOLOGY 



vertically. If, however, h be so placed that its slit is horizontal (fig. 71), the 

 vibrations will be extinguished on reaching b, and the string between b and d 

 will be motionless. 



Fig. 71. 



c here represents a source of light, and the vibrations of the string the 

 undulations which by the nicol's prism a are polarised so as to occur in one 

 plane only ; if the second nicol or the analyser b is parallel to the first, the 

 vibrations will pass on to the eye, which is represented by d ; but if the planes 

 of the two nicols are at right angles, the vibrations allowed to pass through 

 the first are extinguished by the second, and so no light reaches the eye. In 

 intermediate positions, b will allow only some of the light to pass through it. 

 It must be clearly understood that a nicol's prism contains no actual slits, 

 but the arrangement of its molecules is such that their action on the particles 

 of aether may be compared to the action of slits in a diaphragm to vibrations 

 of more tangible materials than aether. 



The Polarising Microscope consists of an ordinary microscope with certain 

 additions ; below the stage is the polarising nicol ; in the eye-piece is the 

 analysing nicol ; the eye-piece is so arranged that it can be rotated ; thus the 

 directions of the two nicols can be made parallel, and then the field is bright ; 

 or crossed, and then the field is dark. The stage of the microscope is arranged 

 so that it also can be rotated. 



The polarising microscope is used to detect doubly refracting substances. 

 Let the two nicols be crossed, so that the field is dark ; interpose between 

 the two, that is, place upon the stage of the microscope, a doubly refracting 

 plate of which the principal plane is parallel to the first prism or polariser 

 the ray from the first prism is imaffected by the plate, but will be extinguished 

 by the second ; the field therefore still remains dark. If the plate is parallel 

 to the second nicol the field is also dark ; but in any intermediate position, 

 the light will be transmitted by the second nicol. In other words, if between 

 two crossed nicols, which consequently appear dark, a substance be interposed 

 which in certain positions causes the darkness to give place to illumination, 

 that substance is doubly refractive. How this takes place may be explained 

 as foUows : — 



Let NjNj (fig. 72) represent the direction of the principal plane of the 

 first nicol, and N^Nj that of the second. They are at right angles, and so 

 the ray transmitted by the first will be extinguished by the second. Let 

 PP represent the principal plane of the interposed doubly refractive plate. 

 The extraordinary ray transmitted by NjNj vibrates in the plane NiNp and 

 falls obliquely on the plate PP ; it is by this plate itself split into two rays, 

 an ordinary and an extraordinary one, at right angles to one another, one 



