Micro; 10 



passing through the specimen is changed by local variations in the 

 optical path (thickness X refractive index) in the specimen. Changes 

 in the other beam of light, which is called the reference beam, depend 

 upon the average optical path of the region to one side of the specimen 

 in the shearing system or upon the average optical path of the specimen 

 and the region around it in the double focus system. 



After passing through the microscope slide and specimen, the two beams 

 of polarized light are recorabined by another thin plate of birefringent 

 material on the front of the objective. The recombined beams are then 

 focused and magnified by the objective lenses, although remaining polar- 

 ized into beams at ninety degrees to each other. Passage of these beams 

 through the quarter-wave plate changes them into right and left-handed 

 circularly polarized light. In effect, the result of combining the two 

 polarized beams is a single beam of polarized light lying in a polarized 

 plane. The direction of orientation of this plane depends on the phase 

 difference between the two combined circularly polarized beams. The 

 analyser of the instrument is a calibrated plate of polarizing material 

 and is calibrated to measure the direction of the beam. Thus, phase 

 differences in the specimen with respect to the reference beam can be 

 measured. 



To obtain results with this type of instrument it is necessary to obtain 

 light entering the system all in one phase. This is done by making 

 adjustments of the substage condenser which produce a series of spectra 

 much as seen as Newton rings between two closely adjacent pieces of 

 glass or much\like the rainbow colors seen in an oil slick. By suitable 

 adjustments, a single color (single phase) of the appropriate spectrum 

 is expanded wide enough to fill the field of the objective lens. Thus, 

 the reference beam and the beam which passes through the specimen begin 

 their entry in the identical phase. The other aspects of the instrument 

 make it possible to detect even rather slight changes produced by the 

 details in the specimen. 



The interference microscope has been developed and utilized mainly by 

 biophysicists. It has been regarded principally as a measuring tool, 

 and, perhaps, its greatest value lies in this aspect. However, many 

 users will find that otherwise invisible subject contrasts can be seen 

 in different and variable colors as though the structural details were 

 differentially stained. The colors are related to the optical path in 

 the specimen, and with experience one can gain some information from the 

 color differences or changes. So, like the phase contrast microscope, 

 the interference contrast microscope has great potential for the study 

 of living subject matter. 



The shearing system of condensers and objectives is preferred for 

 measurement purposes, and I recommend this system for studying nematodes 

 whether or not quantitative work is intended. In the shearing system 

 the reference beam is displaced to one side of the specimen. Therefore, 

 advantage can be taken of the fact that most nematode specimens are 

 relatively narrow in diameter as contrasted with a specimen or structure 



