THE DETERMINATION OF REFRACTIVE INDEX 
241 
that the cover-glass shall lie parallel to the object slide, or if not 
truly parallel, that the measurement of the depth of the cell and 
that of the displacement of the image be made at the same point. 
Since there is always a thin film of liquid between the cover-glass 
and top of the cell, the value for A should be determined with 
the cell filled and all data necessary for the computation be made 
at once. 
This method gives values to three decimals for n of which two 
places at least will be correct and the third not far from the true 
value. 
Correct results are more easily obtained with red or yellow 
light than by ordinary daylight. 
In the absence of a suitable cell, a simple container for the 
liquid may be made from narrow strips of glass cut from an 
ordinary thin object slide and laid as shown in Fig. 141. These 
strips of glass are easily cut with a glazier’s 
diamond or with the sharp end of a file. 
The liquid to be studied is allowed to drop 
into the opening between the glass strips, and 
the cell upon being covered remains filled by 
capillarity. The cover is gently pressed down fjg. 141. 
and the excess of liquid removed with absorbent 
paper or a piece of drawn-out glass tubing. Since there is a 
film of liquid in this case between both the upper and lower 
surfaces of the cell walls, considerable care must be exercised 
to avoid serious error. In any event the results are to be 
regarded as approximations only. 
Method 2 . Determinations of Refractive Index from a Curve 
Plotted from Readings Obtained with Liquids of known Refractive 
Index} 
In this method a scratched slide or the method of projected 
image may be employed. The latter because of its greater con¬ 
venience will be found the better. It is unnecessary to know 
the depth of the cell, the thickness of the cover-glass or the true 
values of the graduations on the fine adjustment. . 
1 Suggested to the author by F. E. Wright, Geophysical Laboratory, Washing¬ 
ton, D. C. 
