INTKHKKKKIVCE MICROSCOPY 



jectivc. It is important to avoid impurity 

 radiation in quantitative work on long, 

 narrow samples such as fibers. A nimiber of 

 excellent infrared spectra made with the 

 microscope attachment have been published, 

 including tissue sections, single fibers (17- 

 20 fi), licjuids in short lengths of AgCl capil- 

 lary tubing (3), and single crystals (6, 8, 9). 

 The determination of optical constants in 

 the infrared especially for metals is a special 

 contribution of Beattie and Conn (1, 2). 

 Military surplus infrared image electron con- 

 verter tubes continue to find application for 

 the infrared examination of opaque dyes and 

 pigments, and silicon ingots. In the latest 

 work according to news reports the infrared 

 microscope is adaptable to the examination 

 of substances which can be differentiated on 



the luisis of their specific infrared emissivity 

 and reflectivity, as well as absorption (7). 



REFERENCES 



1. Beattie, J. R., Phil. Mag., 46, 235 (1955). 



2. Beattie, J. R. and Conn, F. K. T., Phil. Maq., 



46, 222 (1955). 



3. Bloxjt, E. R., Parrish, M., Bird, G. R., and 



Abbate, M. J., J. Opt. Soc. Am., 42, 966 

 (1952). 



4. Coates, V. J., Offner, A., and Siegler, E. H., 



/. Opt. Soc. Am., 43, 984 (1953). 



5. Eraser, F. D. B., J. Opt. Soc. Am., 43, 929 



(1953). 



6. LowENTHAL, S., Rev. Opt., 34, 29 (1955). 



7. Maresh, C, Coven, G., and Cox, R., Anal. 



Chem., 30, 829 (1958). 



8. NoMARSKi, G., Rev. Opt., 34, 29 (1955). 



9. NoRRis, K. P., /. Sci. Inst., 31, 284 (1954). 



G. L. Clark 



Interference microscopy 



FIBERS (TEXTILE). See GENERAL MICROSCOPY, 

 p. 343. 



INDUSTRIAL RESEARCH, APPLICATION TO. See 

 GENERAL MICROSCOPY, p. 363. 



INSTRUMENT CLASSIFICATION AND 

 APPLICATIONS 



Interference microscopy is a techiiicjue for 

 the microscopic observation of specimens, 

 whereby two superimposed fields of view are 

 presented to the observer. One field con- 

 tains an image of the specimen and is called 

 the "image field"; the other (the "reference 

 field") differs from the image field in some 

 way, but the two fields are mutually co- 

 herent at every point. Contrast is produced 

 by the effect on the interference phenomena 

 of the path-differences caused by the optical 

 thickness (for transparent objects) or surface 

 contour (for reflecting objects) of the speci- 

 men. 



It will thus be seen that interference mi- 



croscopy makes use of the same properties 

 of the specimen as does phase contrast mi- 

 croscopy. The advantages of the interference 

 method are that certain artefacts (the 

 "phase-contrast halo") are avoided, that 

 measurements of path difference can be 

 made precisely, and that the nature of the 

 contrast can usually be altered by varying 

 the overall path difference between the two 

 interfering fields to obtain the best condi- 

 tions for the detail being studied. A further 

 advantage is that in some types of inter- 

 ference microscopes the aperture of illumi- 

 nation is not restricted. 



Classification of Instruments 



It is convenient to classify interference 

 microscopes by the way in which the image 

 and reference fields differ from each other. 

 Three classes can be recognised, as follows: 



I. The reference field is formed by light 

 which has had no contact with the object. 



IT. The reference field contains an image 



412 



