INTERFEROMETRIC METHODS 



be polarized in the same plane ; interference had some practical use. The one designed by- 

 is impossible if the planes are at 90° to one Brillouin in 1903 (in complete ignorance of 

 another. For other relative orientations, all Jamin's work) was used to detect extremely 

 degrees of contrasts of the fringes are ob- small displacement of a galvanometer frame 

 served. supporting the second crystal. In 1930 

 There is, however, one important excep- Lebedeff (Optical Inst, of Leningrad) built 

 tion to the general laws above. If a pencil of another polarization interferometer in which 

 light is rectilinearly polarized, then split by the two beams were not exactly separated 

 one of the methods indicated herein, the re- but only slightly so. Used conjointly with 

 sultant beams are polarized at 90° to one a polarization microscope, the apparatus 

 another, as is well known. Yet, when they served for the measurement of the refractive 

 are brought again to coincidence on a common index of very small crystals suspended in a 

 plane, interference is still possible. Such liquid, and for the study of heterogeneities 

 splitting of an incident ray may be obtained in solution. 



by utilizing the phenomenon of double re- The use of the Jamin apparatus as a pre- 



fraction in certain crystals. If the incident cision colorimeter must be mentioned. The 



light is white, in this case the emergent rays complete modus operandi was developed 



are no longer so. Furthermore, one must be by Barchewitz (87). 



cognizant of the fact that one of the emer- An interference microscope built by F. H. 



gent rays suffers a retardation, relatively to Smith (88) and utilizing a plurality of bi- 



the other, of one-half wavelength. refringent coaxial elements constitutes an 



In an instrument based upon this princi- interesting extension of the principles em- 



ple and built by Jamin in 1868 the separation bodied in Jamin's polarization interferome- 



of the rays was obtained by two very large ter. 

 spar crystals. The original memoir had been 



completely forgotten when, in 1934, A. Cot- Amplitude Splitting Interferometers 



ton (86) accidentally discovered the original The prototype in this class is the well 



apparatus in a pile of discarded antiques. known Michelson Interferometer. The two 



The essential feature of this instrument is instriunents built by Sagnac (89) in 1911 



that it can produce both localized and non- and 1914 are, similarly, based upon the 



localized fringes. If the incident light is ex- property of semireflecting "beam splitting" 



actly parallel and if the two crystals are also surfaces. 



exactly parallel, the two beams of light are The Michelson beam-splitting system has 



superimposed and one observes only a uni- been modified in numerous ways to satisfy- 



form color depending upon the orientation particular technical requirements. One of its 



of the half -wave plate. Non-localized fringes chief advantages is the wide separation ob- 



appear if the observation is made with a tained. Usually the two coherent beams are 



telescope adjusted adinfinitum, according to at 90° to one another. 



Raveau's rule. If the two crystals are not The "Zeiss-Opton" interference micro- 



exactly parallel and if the light source is a scope is a good example of an instrument 



very small pin hole, the fringes become visi- based upon the amplitude-splitting principle, 



ble on a distant screen, without any tele- It utilizes a collimated parallel incident light . 



scope. In addition, there is another system The interferometer attachment to the Chap- 



of localized fringes, near the emergent face man research polariscope is based on the 



of the last crystal, and due to the defects same principle. The Twyman-Green inter- 



of the latter. ferometer and the so-called Penn-modified 



Instruments built on this principle have Twyman instrument, utilize collimated in- 



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