274 



WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



Fig. 5. 



slight angular differences in inclination between incident rays increases 

 with the focal length of the objective used. 



Method 5. For this method a plane-parallel plate of highly refract- 

 ing lead glass (?7,Na = 1.92) with one plane polished and one plane matt 

 finished edge inclined at an angle of 60° is required. This plate (20 x 



20 X 4 mm.) after grinding is cut into two halves 

 at right angles to the beveled edge (fig. 4) . The 

 two halves are then placed in contact with the 

 polished edge of the one half above the matt 

 ground edge of the second half. A drop of 

 liquid between the two plates is held in place 

 by capillarity and its refractive index ascer- 

 tained by observing the position (16 mm. ob- 

 jective, Bertrand lens, positive eye-piece with 

 micrometer scale) of the limiting refracted ray 

 between the light and dark fields as indicated 

 in figure 5. This limit between the light and 

 dark halves of the field is sharply imaged in the 

 plane of the micrometer scale by means of which its position can be 

 read off directly. 



The scale is calibrated empirically once for all by the use of substances 

 of known refractive index. Three substances, which can be had in 

 powder form and which melt at low temperatures, can be used for 



calibration purposes because of con- 

 stancy of refractive index for sodium 

 light; they are "Kollolith, WNa = 

 1.5354; benzophenon, WNa = 1.598 

 and piperine, nNa = 1.682. Liquids 

 of known refractivity can also be 

 used but, unless controlled b}^ meas- 

 urements on a refractometer, the re- 

 fractive indices of commercial liquids 

 are not sufficiently uniform to serve 

 as calibration standards. After the 

 scale has been calibrated and the 

 points plotted, a curve can be passed 

 through the observed points, as indi- 

 cated in figure 6, which represents the 

 values actually observed by the writer 

 on liquids of known refractivit}^ 

 The curve is a small part of a sim- 

 ple sine curve and is sufficiently ac- 

 curate for refractive index determina- 

 tions to the third decimal place. The measurements were made with 

 a 16 mm. Zeiss apochromat objective, Bertrand lens and 0.1 mm. coor- 

 dinate scale in the focal plane of a positive eye-piece. One-quarter of 

 a division of the scale could be easily read and was equivalent to 0.001 

 in refractive index. With a higher power eye-piece or a moving micro- 

 meter eye-piece still more accurate readings can be made and more 



1.50 1.60 



REFRACTIVE INDEX 



1.70 



Fig. 6. 



