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 up 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. 
