REFRACTION OF LIGHT, REFRACTOMETRY AND INTERFEROMETRY 



illumination. This generally neglected prob- in the plane of the fringes, a photocell and a 



lem is not as simple as often assumed. How- pulse-type amplifier followed by a mechan- 



ever, it is possible to formulate a few recom- ical counter. The Ferranti system (108), al- 



mendations of general value. The physical though designed for counting the "moire" 



structure of the light source is important, fringes produced by crossed gratings, utilizes 



Usually it is the exit aperture in the source this principle. Direction discrimination is 



housing which acts as the real, primary radi- achieved by simultaneously inspecting two 



ation source, and not the heated filament of points on the pattern separated by an odd- 



the lamp. The constructor of an optimum number of qviarter-wave lengths. An elec- 



condenser must take this fact into considera- tronic binary counting system has also been 



tion. Surprisingly, optimum performance is successfully operated with a modified Mich- 



not necessarily realized when the source is elson interferometer by Peck and Obetz 



exactly focused on the entrance aperture of (109). Such double scanning systems present 



the instrument. certain difficulties. If both the amplitude of 



The so-called field-type illumination the fringe shift and its direction must be 



(source imaged onto the collimator lens) is recorded, it is necessary to arrange the ap- 



essential for most photographic applications, paratus so that a photo-electric signal pro- 



For photoelectric measurements, it is pre- portional to the first quantity is produced, 



ferable to use the Twyman -Simeon mount- Then a phase modulation related to the 



ing, as discussed by Stockbarger and Burns second quantity (direction) is introduced. 



(106). However, if a monochromator is in- The operation amounts to effect an auto- 



terposed between the source and the measur- matic differentiation of the fringe energy 



ing instrument, the desired spectral resolution distribution curve, including extraction of 



enters into the calculations and the general the differentiation sign. The desired result 



solution requires that only approximately can be achieved with two photomultiplier 



one-half of the monochromator collimator tubes in a bridge circuit, as proposed by 



diameter be utilized (thus sacrificing much Ingelstam (110). A general approach to the 



of the available radiant power) . In this last problem was discussed in a previous publica- 



case, much is regained by substituting mir- tion (111). Considerable instrumental sim- 



rors for lenses in the illumination system. A plification results from the use of a single 



more complete discussion, with relations de- radiation receptor and a scanning mecha- 



rived from the application of the spatial nism operating at a fixed frequency. Similar 



frequency analysis method, was given in a advantages accrue by modulating the output 



previous publication (107). amplified signal in relation to the direction 



of fringe shift. 



Continuous Recording in Interferential ^he fringe interpolator developed at the 



Refractonietry 



National Bureau of Standards (112) for use 



The practical value of interferometric with a Pulfrich-type interferometer intro- 

 measurements is greatly enhanced by the duces a modulation at the fundamental fre- 

 availability of automatic, continuous record- quency of a tuning fork on one of the light 

 ing methods. Photographic and cinemato- beams. A similar system is employed in the 

 graphic recording have long been successfully Glennite Co. interferometer, which thus 

 used. However, deciphering the films is a could be easily adapted for continuous re- 

 time-consuming procedure. A considerably cording. Other modulation methods must be 

 faster procedure utilizes electronic fringe considered for special applications. For in- 

 counters, several models of which exist. A stance, magnetostriction of the short nickel 

 simple system includes only a scanning slit cylinder (attached to one of the elements of 



514 



