Fossil Aulacodiscus from St. Laurent (images added)

Beatsy · Post by **Beatsy** » Fri Dec 26, 2014 1:56 pm

I just finished cleaning a sample of raw diatomite from St. Laurent, France. There weren't many intact forms in the largest fraction (the material held back by a 175um mesh) but here's a nice Aulacodiscus reticulatus I picked out.

I'd hoped to get all details in a single picture, but it proved impossible even using a deep stack taken with a 40/0.95 planapo objective. The sharp detail of the pores kept obscuring the smoother overlying reticulation (which looks a bit like the skin of a cantaloupe).

I included 3 single-shot images taken at lower resolution with the condenser aperture closed down somewhat. This shows the reticulation more clearly, First inset is focused on the top of the valve, second halfway through and third near the bottom.

Guess it just goes to show you can't always capture everything in a single image, even with stacking, and particularly with transparent subjects like diatoms.

Pau · Post by **Pau** » Fri Dec 26, 2014 3:14 pm

Excellent!!!

Cactusdave · Post by **Cactusdave** » Sat Dec 27, 2014 2:31 am

Really nice diatom find, and a fine image. The problem of trying to stack and display internal structure which is overlaid with a semitransparent, but structured surface is a perennial one. I have encountered it a lot with slide mounted small insects like bird lice.

Wim van Egmond · Post by **Wim van Egmond** » Sat Dec 27, 2014 3:28 am

It is still a very nice result. Stacking is hardly ever perfect. But have to tried to photograph this subject with dark field? It may be hard with this NA objective but if you have an objective with an iris, or perhaps a lower NA objective.

Wim

Pau · Post by **Pau** » Sat Dec 27, 2014 4:01 am

Wim van Egmond wrote:... But have to tried to photograph this subject with dark field? It may be hard with this NA objective but if you have an objective with an iris, or perhaps a lower NA objective.

There should not be problem with the Zeiss oil dark field Ultracondenser 1.2-1.4, I use it with my 50/1.00 NPL Fluotar

Beatsy · Post by **Beatsy** » Sat Dec 27, 2014 4:23 am

Thanks all for the comments.

I had a look in darkfield, but it seemed far too "busy" or cluttered for that - overlying details obscured underlying ones even more. I could only use a 16x for that though. I used to have the ultra darkfield condenser, but it went missing during a house move. Two years on and I still occasionally search old boxes and obscure corners in the hope it might show up, but it never does

On an interesting side note, I found an old plate of diatom sketches that had two views of A. reticulatus drawn. One focused on the surface of the valve, one on the pores. From memory, I think it was a plate by Moller - so it seems I'm in good company with what appears to be a (very) long standing issue

carlos.uruguay · Post by **carlos.uruguay** » Sat Dec 27, 2014 5:27 am

Really interesting

anne · Post by **anne** » Sat Dec 27, 2014 8:33 am

Hello,

really very nice stack of a very difficult diatom! Thank you for this great image.

Beatsy · Post by **Beatsy** » Sat Dec 27, 2014 2:07 pm

Looks like "more NA" is the answer here. I oiled a planapo 100/1.3 and an NA1.4 condenser lens to the slide, added a touch of DIC and rattled off two stacks of the same diatom. The reticulation shows very clearly.

As I'd gone to the trouble of faffing with immersion oil, I took a stack from a large Coscinodiscus sp. mounted on the same slide (it's about 320um diameter). I considered these to be a rather "boring" genus, but was surprised at the level of extra detail that showed up. It's pretty much at the resolution limit of this setup, but I'm tempted to break out a deep violet filter and have another go with that. But too many new strews to search at the moment, so that will have to wait for a slow day

Olympusman · Post by **Olympusman** » Sat Dec 27, 2014 2:23 pm

##### Beasty, these oil immersion stacks are off the charts. You dah man.

Mike

carlos.uruguay · Post by **carlos.uruguay** » Sat Dec 27, 2014 2:57 pm

Awesome!!
A fractal design remains to infinity!
Thank you for sharing it

Cactusdave · Post by **Cactusdave** » Tue Dec 30, 2014 2:58 am

The first of those shots with the oiled condenser is very very nice, just about as near to SEM as I've seen, and really on the edge of possible light resolution. Again I wonder if you have tried to make stereo pairs with these?

santiago · Post by **santiago** » Sat Nov 24, 2018 3:38 am

the oiled planapo images showing the reticulation are amazing!! the lighting and shading draws the shapes so nicely