Microscopy Forum

Every now and then I feel I should try something a bit more  microscopy. Such as plant microscopy. Today was the day.
I had the feeling many times when I had a stereo microscope and was moderately successful. The stereo microscope is gone, replaced by a polarizing microscope with umpteen features. But my polarizing microscope does not have  top lighting or a filter holder below the condenser so the following experiment is both revealing and funny.

My subject was a petal from a dried Helichrysum or Straw Flower.

My slide preparation was crude. Place a drop of Super Glue on the slide. Lay a petal on the glue. Put another drop of glue on the petal. Lay a cover slip on the specimen. Press gently with another slide to try to get rid of as many  air bubbles as possible.

With the polarizers at extinction, the petal was quite visible.
As I have discovered in the past, what I think is small is quite large under a microscope. It took 19 separate photos  to make a photo of the whole petal.

2x objective with crossed polars.

Helichrysum petal2  L5X_10_2x 19i

A lot has been written about using darkfield stops to better show details. Well, my microscope does not have a filter holder and this is where the fun starts.
Not having a filter holder I held the blank compensator (basically a black bar about 15mm wide by 80mm long.) under the condenser and pressed the shutter release.
It worked, sort of. There is more detail. I was so pleased I save a full size TIFF file to use for a future shark piece. You do see the resemblance to a shark.

Helichrysum petal2  L5X_30 2x black bar

I do have darkfield stops for a different microscope. I placed an 18mm stop over the light source lens and got a black screen. Believing I had forgotten to switch out the top condenser lens, I turned the condenser lens knob and got this beautiful photo.

Helichrysum petal2  L5X_33 2x no top c DF18

Now an 18mm stop is far too large for a 2x obj. so I switched to the 10x obj.
Crossed polars, 10x obj.

Helichrysum petal2  L5X_40 10x

This is what I saw with the 18mm stop lying on the light source.

Helichrysum petal2  L5X_41 10x DF18

The moral of the plant microscopy story is: Use a 2x objective and carry a big black bar.

The microscope used for this experiment is  a dedicated petrology or polarizing microscope.

There is no filter holder below the condenser, just the polarizer.
The lighting system is a 100w halogen with a field diaphragm.
Normally with a 10x or higher objective I would use Köhler illumination, but with a 5x and lower objectives the field diaphragm acts as the condenser diaphragm.  Köhler illumination is not valid below 5x.

The blank compensator bar held below the polarizer worked, sort of, while the darkfield stop on the light source did not.

I assume the reasoning for no filter holder below the condenser is there is little demand for it in the petrology  field.

P.S.

From the Olympus web site on Köhler illumination.

"Modern microscopes are equipped with specialized substage condensers that have a swing-out lens, which can be removed from the optical path for use with lower power objectives (2x through 5x). This changes the performance of the remaining components in the light path, and some adjustment is necessary to achieve the best illumination conditions. The field diaphragm can no longer be used for alignment and centering of the substage condenser and is now ineffective in limiting the area of the specimen under illumination. Also, much of the unwanted glare once removed by the field diaphragm is reduced because the top lens of the condenser produces a light cone having a much lower numerical aperture, allowing light rays to pass through the specimen at much lower angles. Most importantly, the optical conditions for Köhler illumination no longer apply."