IM)ISIHI\L HKSEAKCH 



trast accompanying an increase in the light intensities observed in the field, as in 

 numerical aperture. Frequently there must phase microscopy. Interference microscopy 

 be a compromise toward lower numerical is very new and has hccii used principally by 

 aperture, this time to achieve desired con- the biologist, but should find application in 

 trast. Optical aids to increase contrast have industry, especially in the study of trans- 

 been de^'eloped and have pro\'ed of special parent films comprised of structures ha^'ing 

 interest to the biologist whose living prepara- small differences in refractive index or thick- 

 tions are best studied in that condition, ness. 



Nevertheless, the industrial microscopist also Polarized-light microscopes are useful in 



has a persistent need for these special tools, increasing contrast and in making optical 



which include phase contrast, interference, measurements on crystalline and paracrys- 



and polarizing microscopes, and filters. talline subjects. Differences in orientation of 



Phase, Interference, and Polarizing crystalline and fibrous materials are es- 

 JMicroscopes. The phase microscope takes pecially well revealed. A polarizing micro- 

 advantage of the optical-path differences scope is unicjue in that specimens are studied 

 within the specimen which result in the while illuminated with polarized light. A 

 existence of phase differences among the polarizer, transmitting light vibrating in one 

 light waves transmitted by the various por- plane, is usually placed between the con- 

 tions of the specimen. Further phase changes densing lens and the source of illumination, 

 are introduced in the light passing through and an analyzer, also of polarizing material, 

 the optical sj^stem and add to the phase is located in the tube between the eyepiece 

 differences created by the specimen and and objective. Generally the polarizer and 

 thereby render the object visible (3). Briefly, analyzer are used in the crossed position so 

 the diffracted rays are altered by a phase- that their respective planes of vibration are 

 retardation coating at the back focal plane perpendicular. In this position, the field of 

 of the objective, the intensity of the un- view is completely dark, and the sample is 

 deviated light is reduced by a filter in the visible only by virtue of any effect it might 

 form of a ring at the back focal plane of the have on the original plane of vibration of the 

 objective. The deviated beam interferes with light with which it is illuminated. Figure 1 

 the diffracted rays at the focal plane of the shows an image produced by anisotropic 

 eyepiece to form an image, the contrast of crystals under polarized light. Optical 

 which is derived from these exploited phase properties of substances have been measured 

 differences. Special phase microscopes, both for many years by means of polarized light, 

 of transmitted- and vertical-illumination Mineralogists, chemical microscopists, and 

 types, or components for conversion of exist- crystallographers use ciualitative and quanti- 

 ing microscopes, are available commercially tative optical data such as refractive index, 

 from several manufacturers. The resolving birefringence, optic sign, extinction, etc., as 

 limits of phase objectives are not as high as an adjunct to other information in the identi- 

 comparable objectives for conventional fication of crystalline materials. Although 

 bright-field work, but the increase in con- these methods are highly useful in industrial 

 trast may overshadow the lack in resolving studies, no attempt will be made to present 

 power. any detailed information in this area. Ex- 



The interference microscope, as the name cellent coverages of these techniques appear 



implies, produces contrast in the image by in the literature (4, 5). Polarized-light 



optical interference. Contrast is apparent microscopy is eciually applicable to either 



because of differences in color (when white transmitted or ^'ertical illumination. In 



light is used) rather than in differences in metallographic and ceramic research, the 



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