INTERFEROMETRIC METHODS 



responding dn) and the angular rotation ob- must be introduced. Certain combinations 



served. This is obtained by means of a of refractive dispersion value result in the 



"tangent screw" mechanism. Fairly good presence of several achromatic fringes 



linearity can thus be obtained within a rota- ("sprung" phenomenon). This is hkely to 



tion range of about 20° on each side of the occur with strongly colored fluids with sharp 



zero-position. The range of measurements absorption bands. A complete discussion of 



depends primarily upon the angular setting such disturbing dispersion phenomena was 



of the plates. An increased range can be given by Ollivier (103). Corrective methods 



achieved by the introduction of thin sta- have been proposed from time to time. Even 



tionary plates, in a manner amounting to a when only one achromatic fringe is present, 



true optical zero-suppression method. Grunwald and Berkowitz (88) remark that 



The optical devices available for pointing the compensator dispersion still introduces 



the fringe system in its zero-position re- a small residual error whose exact correction 



quire some comment. In general, a microme- is very involved. The practical correction 



ter offers definite advantages over a simple method proposed long ago by the writer (97) 



cross-hair or other fiduciary reference marks still remains valid and sufficient in a large 



(Brillouin) (87) provided the correct tech- number of cases. 



nique is employed. Pointing may be effected The developments summarized above are 

 either with a telescopic ocular, or with a low- embodied, with utmost condensation, in a 

 power microscope. Photography of real fringes small low-cost portable interferometer built 

 does not present unusual problems. The ac- by the writier in 1953 — the so-called orthop- 

 curacy of the pointings is always limited by tic microscope interferometer (104). This 

 the sine distribution of energy in the fringes, last publication and a patent (105) give the 

 among other factors. It is usually limited to basic principle employed, the details of con- 

 3^ of the interfringe spacing. Increased ac- struction and a complete functional analysis. 

 curacy is obtained if a small slit is placed The instrument utilizes the microscope's 

 in the focal plane of the viewer and the bundle of marginal rays, which are parallel 

 hght is modulated at 15 cps. Such a flicker and sufficiently coherent — hence, the in- 

 method, previously described (102) raises the strument's name. The device fits on any 

 reproducibility of the pointings to 3-^0 of oii6 commercial microscope. These same publi- 

 fringe. Still greater accuracy is possible by cations also offer a practical approach to the 

 electronic recording (v.i.). The accuracy of correction of the errors inherent in most in- 

 final pointing of zero-position of the fringes terferometric measurements on liquids: (1) 

 is a problem quite apart from that of making the fluid layer thickness, (2) the fluid corn- 

 accurate visual refraction measurements, partment parallelism, (3) the error on wave- 

 These measurements involve tracking the length determination, (4) the variation of 

 fringes from their extreme shift position back apparent color temperature of the light 

 to the zero-position. This latter problem is source when polychromatic measurements 

 complicated in monochromatic Hght by the are contemplated, (5) the pointing errors, 

 uniform appearance of the fringes : the "mid- (6) the compensator calibration error, (7) 

 die reference fringe" is unrecognizable. In the scale reading error, (8) the effect of 

 polychromatic light, the middle "achro- ambient temperature changes, and (9) the 

 matic" fringe is theoretically recognized by role of temperature gradients within the 

 its symmetrical chromatism. In practice, this measuring compartment, 

 aspect is marred by the disturbing effect of 

 the spectral dispersion of the compensator Illumination Systems 



plates and of the layer of examined fluid, Optimum performance of all types of re- 



and rather complicated correction formulas fractometers depends a great deal on correct 



513 



