Grenzdunkelfeld

[Muschelbluemchen]

@rene

indeed Leo, can you explain again what it is you want to say? IMHO there is not much that could be added in Peter's image of Pleurosigma.

Perhaps this will explain what I meant:


out of the book: "Das Mikroskop in der Schule" - Hartmut Dietle.
Please focus on "In der Praxis gibt es Objektive, die in der Schale von Pleurosigma zwei Streifensysteme auflösen."

mit freundlichen Mikrogrüßen
leo

[Rene]

Streifen, Streifen, dots I want to see! 
And they are there in the image, no problem.

I'm always having this image from Roderich Zeiss in my head when thinking about Pleurosigma:



And that's from 1891!

Best wishes, René

[Holger Adelmann]

Hi Rene,

we need to compare apples with apples.
Peter did not show a brightfield image as you were referring to with this surely extraordinary 1891 image, but was more after showing the special image effects of a borderline darkfield setting with the Heine condenser.

If you use a good recent brightfield objective you can compare the result to the old photomicrograph you have shown.
I attach a brightfield picture of Pleurosigma angulatum in air taken with a Leitz Pl Apo 63/1.40 (160) in white light, of course contrast and sharpness can still be optimised... but they did not have these tools in 1891 in all fairness.... but with experience they could at least develop the picture to a steeper contrast, which was probably done in your example.
This was also the most plane specimen I could find on my P. a. slide, still not quite as flat as the one in your old picture.

I am sure when it comes to resolving the "dots", Peter will get a result at least as good with his superb Leitz Apo 90/1.40.

Cheers
Holger

[Rene]

Did not have the means in 1891?  

My dear Holger, you haven't had the chance to look into the bible of Microscopy? Here is an excerpt from Carpenter (1856!):




And this is a rather poor reproduction (apologies) taken from Dallinger, 1901. Image by van Heurck, 1891 or 1893, can't remember anymore, wasn't entirely there at the time  
Objective was a 1.6NA (!), with flint glass coverslips and slides (RI 1.72) and preparation in Realgar:




By the way, that's a nice image you showed, taken into account that (if indeed this is a dry mounted slide) the NA of your objective is effectively reduced to 1.0. Contrast wise nothing you could wish for. Just the little dots in the middle of the areolae worry me a bit, an interference effect maybe?? For one thing,  P.angulatum areolae have round openings on the inside of the valve, but slits on the outside, something that was under discussion already at the beginning of the 1900's, far before the electron microscope proved them right.

Anyway, the discussion was on visibility of double stripes or three stripes in the image, with a 40x. I guess now I understand why the horizontal stripes are more difficult to visualize, as this 10x5 um crop shows:

Easy peasy with a 60xoil, now for the check with an achro 40x. Hmm, haven't really got one

Best wishes, René