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 fig. 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. 1 



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. 



