224 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



is theoretically free to vibrate in any plane ; T but after passing through the 

 vertical slit in a, the vibrations must all be vertical also ; if a second similar 

 disc b be placed further on, the vibrations will also pass on freely to the other 

 extremity of the string d, if as in the figure (fig. 70) the slit in b be also placed 

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

 vibrations will be extinguished on reaching 6, and the string between b and d 

 will be motionless. 



PIG. 71. 



c here represents a source of light ; the vibrations of the string represent 

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

 one plane only ; if the second Nicol's prism 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 can also 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 unaffected 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 follows : 



Let N t Nj (fig. 72) represent the direction of the principal plane of the 

 first nicol, and N 3 N 2 that of the second. They are at right angles, and so 



1 The imperfection of the model has been explained in the preceding footnote. 



