wriuht: measurement of refractive index 



271 



We may now consider the methods for the measurement of the 

 refractive index of a single drop of Hquid. 



Method 1. A method based on a new type of hollow prism. (Accurate 

 to the fourth decimal place.) This prism (fig. 1) consists of two optic- 

 ally plane parallel glass plates (10 x 20 x 2 mm.) held in place against 

 a 30° glass prism (10 x 10 mm.) by means of a brass 

 clamp (fig. 1). The liquid is introduced between 

 the glass plates and is held in place by capillarity. 

 The semicircular notch in the glass plate (fig. 1) 

 prevents the liquid from creeping up along the 

 ridge formed by the glass plates, and thence into 

 the prism support. The dotted lines in figure 1 

 indicate the size of the prism of liquid obtainable 

 by this method. It measures over 5 mm. on a side. 



With this prism the Abbe method of measurement 

 ' is used. Autocollimation is obtained by the use of 

 a thin cover glass total reflecting prism^ in the rear ^ u- i 

 focal plane of the field lens of the telescope of a '^" ' 



spectrometer or goniometer. The path of the rays is indicated in figure 

 2. In order that the reflected rays be as intense as possible the two 

 plates of figure 1 are silvered on the back along a narrow strip 2 mm. 

 ^vide as indicated by the shaded area on the front glass in figure 1. 

 The measurements are made, after proper adjustment of the prism, 

 b}^ setting first on the normals to the two plates and then on the re- 

 fracted and reflected beams (fig. 2). In the course of a complete rota- 

 tion of the prism two readings of the angles i and r (fig. 2) are made. 





The refractive index is then computed directly from the usual equation 

 This method is accurate to the fourth decimal place, pro- 



/( — 



sm I 

 sin r' 



vided a spectral line source, as mercury, helium or hydrogen light, is 

 used and the circle of the goniometer or spectrometer is accurately 

 divided. The values obtained by this method should be as accurate 

 as any measurements by the standard prism methods with larger 

 riuantities of liquid. 



- F. E. Wright, J. Wash. Acad. Sci., 3: 2.34. 1913. 



