242 ELEMENTARY CHEMICAL MICROSCOPY 



A suitable cell of approximately the dimensions given above 

 is filled with a liquid of known refractive index, covered with a 

 cover-glass projecting beyond the cell wall. The preparation 

 is so placed upon the microscope stage that an observation may 

 be made through slide and cover glass (e.g., along Mi, Fig. 140) 

 with a sharp focus at the exact level of the upper surface of the 

 slide. Set the fine adjustment micrometer at zero. With con- 

 denser and plane mirror project the image of a suitable scale or 

 screen into the plane of the object slide and focus the image 

 sharply by means of the substage screw without in any way 

 changing the coarse or fine adjustments. Move the cell along 

 until the center of the cell falls in the optical axis of the micro- 

 scope. The image of the screen will no longer be distinct. 

 Focus up with the fine adjustment until the screen image is 

 distinct. Read the fine adjustment. This reading is the dis- 

 placement of image produced in this cell by the liquid of known 

 refractive index. 



Empty, clean and thoroughly dry the cell. Fill with another 

 liquid of known but slightly different refractive index and pro- 

 ceed exactly as before. In this manner calibrate the cell using 

 not less than five liquids ranging from water, n = 1.333 U P to 

 methylene iodide n = 1.76. Plot the data obtained on a large 

 sheet of coordinate paper, conveniently with n as ordinates and 

 displacement units as abcissae. 



If more than one cell is at hand carefully number the cell 

 calibrated and number the curve to correspond with the cell. 



To determine the refractive index of a solution of unknown 

 value, fill the cell and proceed exactly as described above to 

 obtain the displacement of the image in terms of fine adjust- 

 ment units. Having found this value, determine its position on 

 the curve and read off the refractive index corresponding thereto. 



This method is capable of yielding results to the third decimal 

 place and is therefore more accurate than Method i. 



A shallow cell is essential, otherwise the displacement of image 

 will be so great with high refractive indices that many complete 

 turns of the fine adjustment will be required to bring the screen 

 in focus. 



