m.cT0'.i2 



The image details are very conplex indeed, particularly if white rather 

 than monochromatic light is used. One can learn how to interpret these 

 images only through considerable experience and practice. There is very 

 little in the literature for one to go by, except rather complex physical 

 and biophysical reports, and it will probably be a while yet before more 

 general usages are reported upon. One special problem encountered in 

 these feeding studies is the design of a suitable micro-chamber for sus- 

 tained observation. Any variations in thickness of any component of the 

 chamber, media, or spaces in the optical path are detected by the inter- 

 ference contrast microscope and can appreciably effect image interpreta- 

 tion. There is no such problem using the phase instrument. 



Phase contrast is certainly not obsolete. Mellors said that if the value 

 of a technique can be judged by the information gained by its use and by 

 the number of workers to which the technique is generally available, then 

 phase contrast is probably the most valuable single method available to 

 cytologists for the study of living matter. We in phytonematology cer- 

 tainly have not utilized this technique to its fullest capacity yet. 



Fluorescence Mcroscopy 



There is another specialized form of microscopy which enables one to 

 detect substances which are not visible with the ordinary techniques. 

 This method involves utilization of the property of fluorescence. 

 Because the technique has been reported in phytonematology literature, 

 a brief consideration of it may be of interest. 



Some materials react with a beam of light in such a way that light which 

 passes through the material has its wavelength increased. If the light 

 applied is ultra violet light, the reaction with a suitable material 

 results in the production of visible light which is usually colored. I 

 think nearly everyone of you have seen displays of fluorescent minerals 

 and cheiTiicals illuminated with "black light" and showing the beautiful 

 and varied colors emanating from otherwise drab materials. 



Plant and animal specimens may contain substances which naturally fluo- 

 resce to a certain extent when exposed to a beam of ultraviolet light. 

 Or, there may be substances present whose fluorescence can be increased 

 or in which fluorescence can be induced by special chemical treatments. 

 Also, it is possible to use differential or selective stains which have 

 the property of fluorescence. Jixamples of some of these stains are 

 coriophosphine, acridine orange, berberine sulphate, rhodamine B, and 

 auramin. 



The apparatus needed for fluorescence microscopy need not be expensive 

 or complex unless special high intensity ultraviolet light sources are 

 needed. The ordinary microscope will serve, provided the lenses of the 

 objective and the internal coating of the body tube do not themselves 

 fluoresce under the influence of ultraviolet light. 



