[Histonet] Another Microscope Question

[Rene J Buesa]

Laurie:
Köhler illumination is not just observing objects in bright field, it implies the use 2 diaphragms; one near the light source and focused in a way that the diaphragm closest to the light source covers (illuminates) only the area to be observed. With the condenser diaphragm you then increase/decrease the intensity of the light. It is most useful for photomicrography and to obtain the maximum resolution of the numerical aperture (NA) of your objectives.
If you look at the slide side wise, you will see that the illuminated area corresponds only to the working filed of each objective. That is why when using Köhler illumination you have to adjust the light source diaphragm every time you change the objective under use.
What your pathologists are trying to do is just to increase/reduce the amount of light reaching the object by increasing/reducing the distance of the condenser from the object to be observed.
Many years ago and starting in the mid 1920s, Carl Zeiss (and also Ernst Leitz) designed the condensers of their research microscopes with the condenser diaphragm mounted on a slide with a knob. By doing so these condensers could move the condenser diaphragm side wise in a way that the light entered into the condenser not straight from the light source obliquously, and the objects were illuminated side wise. This illumination permitted to observe unstained slides and produced the illusion of "three dimension" and permitted to observe unstained structures. This type of illumination was quite popular in hematology labs to observe blood smears
In 1935 the same Carl Zeiss started to manufacture objectives/condenser sytems using Zernike's principle, and phase microscopy was born.
The method of lowering the condenser used by your pathologists has to produce a very poor quality image.
You would be better off if you cut a piece of black paper half the diameter of the filter ring of your condenser and place it there. 
With this opaque filter the light will get to your object side wise, the same as if you have moved the condenser side wise.
René J.


________________________________
From: "Reilly, Laurie" <laurie.reilly <@t> jcu.edu.au>
To: "Histonet <@t> lists.utsouthwestern.edu" <Histonet <@t> lists.utsouthwestern.edu> 
Sent: Tuesday, July 10, 2012 7:21 PM
Subject: [Histonet] Another Microscope Question

Dear All,
My microscope training has been mainly on brightfield microscopes using Koehler illumination.
Recently, in discussions with Veterinary Pathologists, they have been advocating winding the condenser way down to the field diaphragm when they are examining urine samples. This goes against the microscopy principles that I have been taught and now teach to our students.
Can anyone enlighten me on the value of this practice? Could the same effect be obtained by closing the aperture diaphragm?

Thanks in advance for your wisdom.

Regards,  Laurie.

Mr. Laurie REILLY
Histopathology
School of  Veterinary and Biomedical Sciences
James Cook University
Townsville  Qld.  4811
Australia.

Phone 07 4781 4468


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