WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



277 



IMI 



going methods the micrometer scale is first cahbrated by means of 

 liquids of known refractive indices. This method is slightly more 

 accurate than method 5 but it is less convenient and requires con- 

 siderabl}" more apparatus. It is, however, not encumbered with an 

 error, due to the dispersion of the glass. 



I have also tried to apply the method of total reflection on a single 

 inclined edge of a plate of lead glass, in imitation of the method noted 

 in method 4 but the phenomena obtained were too faint and indistinct 

 to be of service and no satisfactory measurements were made. 



Method 10. Methods of Clerici and Viola. ^ These are based on the 

 difference in refraction between a liquid and a small submerged glass 

 prism (fig. 12). They are noted here simply for the 

 sake of completeness and do not compare in accuracy 

 with the new methods 5 to 9 described above. I 

 have tested Clerici's methods and also Viola's modi- 

 fication of the same and verified Clerici's statement ^^' " 



that his methods are accurate to ±0.005, while Viola's modification 

 is possibly slightlj^ more accurate, namely ±0.004. The methods are 

 simple and easy to appl3^ An improvement is to use a set of fluorite 

 or silica glass prisms (low dispersion), cut at such angles that a large 

 deviation is obtained for slight differences in refractive index, each 

 prism to be cemented above a 0.1 mm. scale. 



Method 11. Method of Piltschikow.^ This method is based on 

 the focal length of a lens system, one of the com- 

 ponents of which is the liquid whose refractive index 

 is to be measured. The liquid is introduced into a 

 hollow piano convex lens and the focal length of 

 the combination measured. To increase the accuracy 

 of setting on the exact focus only the marginal zone 

 is used. This method requires monochromatic light 

 and under favorable conditions should be fairly ac- 

 curate. Modifications of this method Avere suggested 

 by Smith^ and Claj^'" The writer has not had op- 

 portunit}" personally to test the method of Piltschi- 

 kow but it is evidently less simple and accurate than 

 method 5 described above. 



Method 12. Similar results can be obtained by the 

 use of two hollow prisms as indicated in figure 13 or 

 by means of a lead glass plate with two beveled 

 edges (fig. 14). An incident beam of parallel mono- 

 chromatic light and two narrow slits placed symmetrically with re- 

 spect to the two prisms are required. The focus is then the line of 

 intersection of the two refracted beams. The distance of this focus 



Fig. 13. 



^ E. Clerici, Atti della Reale Acadcmia del Lincei, Roma, 16: 1, 336-343. 

 1907; 18: 1, 351-355. 1909; C. Viola, Ibid., 19: 1, 192-197. 1910. 

 « J. Russ. Chem. Soc, 13: 393. 18S1. 

 ^ Amer. Mon. Microsc. Jour., 6: 181-182. 1885. 

 10 Treatise on Practical Light. London, 143. 19n. 



