THE DETERMINATION OF REFRACTIVE INDEX 
239 
adjustment of the microscope. The opening in the cell should 
be not less than twice its depth. A depth of from 0.5 to 2 mm. 
will be found convenient. Select a thin cover-glass of greater 
diameter than the cell and determine its thickness. 
Scratch a very shallow mark at the center and bottom of the 
cell, make a similar mark just outside the cell wall upon the upper 
surface of the object slide. Make a scratch upon the cover- 
glass near an edge. 
Fill the cell with the liquid whose refractive index is to be 
determined. Cover with the cover-glass scratched side down, 
being careful to exclude all air bubbles. Press gently to ensure 
that the cell is just completely full. Remove any excess of liquid 
with pieces of filter paper. We now have a cell in substantially 
the condition shown in the diagram. Fig. 140. 
Place the cell upon the stage of the microscope. Focus care¬ 
fully upon the upper surface of the object slide, using the scratch 
as a guide. Read the position of the fine adjustment. Slide 
the cell along until the projecting part of the cover-glass is in 
the field and focus up with the fine adjustment until the scratch 
upon the lower side of the cover-glass becomes clear and dis¬ 
tinct. Record the reading of the fine adjustment. This reading 
will be the depth DD' of the cell plus an error due to the dis¬ 
placement of image resulting from the refractive index and thick¬ 
ness of the cover-glass. Next focus upon the upper surface of 
the cover-glass. The difference between this reading and the 
previous one will give the apparent thickness of the cover-glass. 
If T equals the true thickness of the cover-glass and t the apparent 
thickness then T — / = x where x is the amount which must be 
subtracted from the reading DD' to give the true depth of the 
cell. This value will usually be slightly greater than the depth 
as determined by a gauge. 
Now push the cell along and again focus sharply upon the 
upper surface of the slide outside of cell and cover-glass. Read 
the fine adjustment. Move the cell until its center approxi¬ 
mately coincides with the optic axis of the microscope, focus up 
with the fine adjustment until the scratch made at the bottom 
of the cell is in focus. Read the fine adjustment. The differ- 
