THE DETERMINATION OF REFRACTIVE INDEX 199 



It is obvious that the cell must be accurately ground in order 

 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 ordi- 

 nary thin object slide and laid as shown in 

 Fig. in. 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 FlG iri 



the cell upon being covered remains filled by 

 capillarity. The cover is gently pressed down 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. 



A number of other more accurate methods for the microscopic 

 determination of the refractive indices of liquids have been 

 proposed, but these require specially constructed prisms, wedges 

 or lenses, or fragments of glass of known index of refraction. 

 For information as to methods, apparatus and accuracy the 

 student is referred to the excellent paper by F. E. Wright, The 

 Measurement of the Refractive Index of a Drop of Liquid, 

 Journal Washington Academy Sciences 4, (1914), 269. 



