Piicro:ll 



which fills the entire field of view of the objective. As the reference 

 beam does not go through the nematode specimen less errors are introduced. 

 With the A/O-Baker instrument the specimen should not be larger in 

 diameter than the central 300 ^ of the field for the lOX objective, 

 160 ii for the UOX, and 27 ^ for the lOOX water immersion lens. 



The interference microscope can measure the optical path differences 

 within reasonably transparent subjects. These difference measurements 

 can be made to an optimum accuracy of 1/300 wavelength of the lit^ht 

 used. The ability to measure optical path difference is in itself not 

 particularly important 3 the great value is derived from conversion of 

 these measurements into a variety of quantitative information. 



Interference microscopy has been available to only a few people for only 

 a few years and much exploratory work is yet to be done. Some of the 

 reported applications indicate that we now have a new measuring tool 

 with a very high degree of accuracy and particuarly suited to the study 

 of living, unstained organisms, tissues, and cells, jixamples of some of 

 the kinds of work that can be done include the making of wet and dry 

 mass determinations without damage to the living cell. Enzyme action, 

 hydrolysis, partial solution, and changing concentrations can be measured. 

 Refractive indices and protein concentrations of cytological components 

 can be determined. 



The question naturally raised is whether or not phase contrast micro- 

 scopy is now obsolete because of the development of the interference 

 contrast microscope. So a brief comparison of the two may be useful in 

 helping you decide on their possible applications to your 01m work in 

 Hematology. Both phase and interference contrast microscopes make 

 apparent the differences in optical path (thickness X refractive index). 

 The magnitudes of these differences can be measured with interference 

 contrast instruments. Gradients of the optical path within the specimen 

 are shown to greater advantage with phase and there is better contrast 

 for edges, discontinuities, and fine structures. Variable contrast 

 control is possible with both interference contrast and certain new 

 designs of phase contrast instruments. The particular advantages of 

 interference contrast lie in the ability to measiire optical path dif- 

 ferences, less of the halo effect as seen with phase, colorless struc- 

 tures are made to appear colored (optical staining effect), and it 

 produces images which are more directly related to the structural 

 densities of the specimen than is possible with phase contrast or with 

 darkfield illumination. 



The fact that phase contrast is not satisfactory except for rather thin 

 specimen materials has no doubt limited its application in phytonema- 

 tology. In my own work, which deals with the effects of feeding of the 

 nematodes on their hosts' cells, this is one reason the initial work is 

 being confined to fungi and roothairs which are relatively thin. Simls^r 

 work being done with the interference microscope has the distinct advan- 

 tages in that the nematodes are not too thick and that quantitative work 

 may be possible. However, it does have some serious difficulties, too. 



