166 ELEMENTARY CHEMICAL MICROSCOPY 



displacement should be the same number of degrees as that of 

 the nicols. In like manner compare a number of different seg- 

 ments of the stage graduations. In all cases several observations 

 should be made at each position, the mean of all the readings 

 being taken. 



Polarization without a Nicol Prism. When employing the 

 hot stage microscope it is sometimes essential to obtain polarized 

 light, yet have the substage kept clear. A polarizer of the nicol 

 or other analogous prism type is obviously impossible. Recourse 

 must then be had to polarization by reflection. A variety of 

 devices have been proposed, one of these is illustrated in the 

 microscope shown in Fig. 7. In this type the light is twice 

 reflected below the stage with the result that the object is 

 illuminated by transmitted plane polarized light. The analyzer 

 may consist of any convenient sort of prism, placed either 

 above the eyepiece or mounted to slide in and out of the body- 

 tube. The best results are obtained from reflections from 

 tourmaline plates but Cheshire 1 has shown that fair results 

 can even be obtained from a thin plate of glass, ground on one 

 side, and blackened upon the ground surface. Light reflected 

 from such a plate is polarized; the maximum polarization is 

 obtained when the angle of the incident light is 56^ degrees. The 

 plate may be mounted permanently at this angle and arranged 

 to slip into the substage ring, or in chemical work involving heat- 

 ing with a flame supported by the substage the plate may lie 

 upon the work table, its angle of inclination being obtained 

 by means of a protractor and the plate held in place by means 

 of plasticine for a temporary mounting. A very simple ar- 

 rangement of the Cheshire plate may then be as indicated in the 

 diagram, Fig. 103, the support being an ordinary object slide, 

 while the polarizing plate consists of a half -slide, ground upon its 

 lower surface by rubbing upon a piece of glass carrying very 

 fine emery and turpentine. After cleaning off the abrasive, the 

 ground surface is blackened. A small mass of plasticine is placed 

 upon the slide and the polarizing plate is pressed down until 

 the proper inclination is obtained as indicated in the diagram. 

 1 J. Quekett Micro. Club, 8, 353. 



