REFRACTION OF LIGHT, REFRACTOMETRY AND INTERFEROMETRY 



cident light, too, but they differ from Michel- The construction of the twin diffracting 



son's device in that they produce a plane apertures presents no difficulties, provided 



wave front resulting in a fundamentally one keeps in mind that there are several 



different performance. particular combinations of aperture width, 



The Lotmar interferometer for the study spacing and focal length for which interfer- 

 of electrophoresis phenomena effects a com- ence is impossible. Fluid cuvettes of the type 

 promise between Jamin and Michelson de- built by Jamin were later modified by Lendel 

 signs. It must be mentioned here that, in (100), but are extremely costly. The cuvettes 

 general plane-reflecting surfaces can be re- of the Kern-Lotmar interferometer could be 

 placed by plane gratings in most instruments adapted to the Rayleigh interferometer, 

 listed above. This was originally done by The type of cell designed by the writer 

 Ramsay (91) for the Michelson interferome- proved the most practical for serial determin- 

 ter. A very extensive study of this new class ations. This design minimizes the effects of 

 of instruments is due to Barus (92). Such temperature gradients and sudden varia- 

 modified devices function as their own tions, the role of glass dispersion, and the 

 monochromator. Indirectly, these experi- errors due to defect of parallelism. These 

 ments extended the field of interferometric cuvettes have been redesigned to allow con- 

 measurements to both the very short and tinuous fluid flow with good frontal charac- 

 the very long wavelengths. The application teristics, for recording purposes. They may 

 to electron diffraction spectra is due to Mar- then be used also for continuous diffusion 

 ton (92-93, 94, 95, 96). In the infrared do- experiments by the method of Bruin (101). 

 main, it is no longer possible to separate the The improved design of fluid cuvettes and a 

 production of interferences from polarization more compact construction of the instru- 

 phenomena, for all "beam-splitters" are ment now make it practical to consider inter- 

 gratings which act as good polarizers. Sev- ferometric measurements for continuous 

 eral infrared interferometers are due to industrial plant monitoring and control. 

 Robinson and a commercial device is avail- Observation of the fringe displacement 

 able from Baird Associates. may be visual or photoelectric (automatic). 



The details of construction of a Rayleigh Visual measurements are best made with a 

 type interferometer were previously dis- magnifier (about 20 X) and an adjustable 

 cussed by the writer (97-98). A device built bifilar micrometer or graticule. Greater ac- 

 accordingly by Jobin-Yvon Co. still enjoys curacy is obtained if, instead of observing 

 the favor of European biochemists. The key the fringes shift one uses a compensator and 

 to success is a correct design of the collima- null method. The original Arago compen- 

 tor. An entrance slit that meets Duffieux sator was made of two movable glass wedges, 

 criterion (99) is still the best from the stand- Its precision was poor. The parallel plates 

 point of fringe contrast. Other factors to be compensator of Jamin represents a remark- 

 considered include the slit length, dioptric able improvement, despite its disturbing 

 power, radius of curvature and focal length chromatism. Rotatable glass prisms are 

 of the lens, spectral transmission and disper- sometime used to compensate for this defect 

 sion, minimum spherical aberration toler- (Haber-Lowe). 



ance, chromatism, surface reflection and Modern interferometers are equipped with 



astigmatism. A slit which is 20 to 50 times two symmetrical, rotatable parallel glass 



larger that calculated by the Schuster factor plates (Rayleigh compensator) permitting a 



is still satisfactory, in the visible spectrum, larger range of measurements with a smaller 



Best results are obtained with small lenses, chromatic error. It is convenient to have a 



for only the diffraction fringe of zero-order linear relationship between the retardation 



is useful in such an instrument. introduced by the compensator (or the cor- 



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