.:0 



obtaining good resiilts with phase contrast. Isolated ceJJ.s or thin 

 structures of the nematodes, such as the cuticle or cyst walls, arc 

 thin enough to be seen to considerable advantage with the phase inicro- 

 scope. Obviously, these are rather limited applications. Later, we 

 ohall consider in more detail possible applications of the phase micro- 

 scope to phytonematology. 



Interference Contrast ItLcroscopy 



Mainly due to the development of the phase microscope and its many 

 successful applications, interest was revived in developing a satis- 

 factory interference microscope. Interferometry has been used for 

 years, particularly for measuring with great accuracy the refractive 

 indices of many substances. Suggestions for interference microscopes 

 date back many years, but only recently have really good instruments 

 been made and are becoming currently available without being custom 

 made. Thus, for practical purposes, the interference microscope is so 

 new that most biologists know but little of how it operates and what 

 it can do. 



It should be pointed out again that the phase contrast microscope oper- 

 ates on the principle of interference of light waves and is essentially 

 a micro-refractometer. 



In the interference contrast microscope, as with the phase microscope, 

 two beams of light originating from the same source are necessary. How- 

 ever, in the interference microscope one of the beams passes tlirough the 

 specimen to become altered in phase and the other beam passes to one 

 side of the specimen or is out of focus at the specimen. VJhen the two 

 beams are recorabined in the image plane they interfere and produce visi- 

 ble effects. If white light is used for the illumination, contrasts in 

 the subject are apparent as variable colors. If monocliromatic light is 

 used, subject variations become visible as differences in light intensity. 



Figure 2 diagrammatically illustrates the optical principle of the 

 interference contrast microscope available in this country (A/0-l3aker). 

 This instrument is fundamentally a polarizing microscope, but modified 

 to produce the necessary two beams of light which, after passage through 

 the specimen, are recombined again to produce a single image in which 

 the interference effects are visible. Light first is polarized into a 

 single plane and passes through the substage condenser. On top of the 

 condenser is a thin plate of material which has the property of bire- 

 fringence; that is, it is capable of splitting the nolsrized beam of 

 light into two separate beams which are plare polarized at ri -ht angles 

 to each other. If the two beams are focused by the condenser so that 

 one beam passes through the specimen and the other passes to one side 

 of the specimen, this is called a "shearing system" (Figure 3b). If, 

 however, one beam is focus ::d at the specimen and blie other spreads 

 around the spocinion and ir> brour,ht to a focus above it, tnis is called 

 the "Louble Focus t^vsLcin" (-'i ',viro 3'! ) • 'I'll- pl^^Tse of the light beam 



