REFRACTOMETRIC APPLICATIONS 



an interferometer) with a pulsed magnetic ous "schlieren" methods, striascopy, the 

 field of about 50 oersteds provides a modula- refraetivity of anisotropic media, that of con- 

 tion amplitude of only a fraction of one tinuously variable media (of importance in 

 fringe. The Kerr magneto-optic effect is also astronomy), measurements at very long 

 usable for this application (Decker (113)). wavelengths in relation to nuclear magnetic 

 The writer has discussed several other modu- resonance phenomena, the refraetivity of 

 lation systems applicable to the Rayleigh semiconductor materials, the relationships 

 interferometer (114). The modulation can be between refraction and production of Cher- 

 directly applied to one of the compensator enkow-Vavrilov radiation at super-light 

 plates. Further improvement is derived by group velocities, phase-contrast microscopy, 

 the use of a feedback servomechanism acting and measurements with the phase-contrast 

 on the compensator to maintain the fringes interferometer and with the three-slit inter- 

 on zero. Recording then merely requires ferometer. The latter methods open the way 

 translating the rotation of the compensator to measurements of relative refractive in- 

 shaft into a corresponding variable voltage dices of the order of 10~^ with possible ap- 

 of suitable magnitude and sign, which can be plications in the field of isotopes analysis — 

 continuously monitored by a commercial a point discussed by the writer in 1939 (117). 

 strip chart recorder. One such system has Refractive index measurements find ap- 

 been fully described (115). The chief advan- plication in many fields of analytical chem- 

 tage is the ability to obtain a continuous istry, and even a reasonably complete bibli- 

 record of fringe shifts in terms of an optical ography of applications is beyond the scope 

 retardation which remains linearly propor- of this article. Crystallographic and min- 

 tional to the corresponding refractive index eralogic applications were briefly mentioned 

 variations. Time variations or refractive dis- in the preceding articles. Refraction offers a 

 persion can be conveniently recorded in this positive means of identifying both organic 

 manner. and inorganic crystalline substances. The 

 In all cases successful recording depends combination of refraction and melting point 

 in great part on the performance of a suitable determination is often very specific in or- 

 amplification system associated with the ganic analysis, in the study of alloys and 

 phase-sensitive circuit. A highly satisfactory melts of many kinds and in the stud}'^ of 

 differential AC amplifier developed for this phase equilibria. Metrological applications 

 purpose and exhibiting great zero-stability, too fall outside the limits assigned to this 

 has been described at length (116). article. It will suffice to mention the possi- 



bihty of measuring — and now recording — 



REFERENCES ,, , , ,• ,• i 



very small angular rotations or displace- 



See references for the second article of this ^lents, such as those of micro-balances, dila- 



series (History of Light Refraction), p. 497. j. j. j.i, i -i-i • i.- 



•^ ^ tometers, thermal equilibrumi or magnetic 



R. JoNNARD balances and reference galvanometers, either 



by a method derived from the prismatic re- 



„^^„, -.r^.,„„,_ .r,r,.,^».r,^..,. fractomctcr principle, or by one of the inter- 



REFRACTOMETRIC APPLICATIONS , .^ , %- • , ^ -n 



lerence methods, l^or instance, C. iJarus 



In the preceding articles the principal applied his modification of Michelson inter- 

 techniques of refractometric measurements ferometer (p. 511) to small mirror rotations, 

 — angle refractometry and interferometry — and the writer once transformed his instru- 

 have been presented. Many other facets of ment into a sensitive "static" galvanometer, 

 such refractometric measurements must nee- The subordination of the refractive index, 

 essarily be omitted, among them the numer- n, to chemical structure was suspected by 



515 



