Do you know the Micscape webpages? Plenty of background info. There's also a dutch microscopy group, check http://www.mikros.nl/

Veel plezier ermee, René

The issue of condenser aperture is interesting. As René says, it is of no importance for phase contrast to have a very high NA condenser. Most Leitz condensers have 0.9 NA for all applications as do some Zeiss.

Just supposing your 100x objective has NA 1.25 (which is typical of achromats) and that you were able to use it at full aperture in brightfield or oblique illumination, then theoretically you should be able to resolve to 0.27 microns using a condenser of aperture 1.25 NA with green light. Using a condenser of aperture 0.9 NA decreases resolution to 0.31 microns. This difference is of no importance whatsover for general observational work and could only matter for the most critical applications.

Indeed, achieving the theoretically maximum resolution would be strongly influenced by specimen preparation, mounting media, optical alignment, etc. Moreover, gaining sufficient contrast to observe what has been resolved is likely the biggest stumbling block.

Although I was aware that using phase stops reduces the effective NA I hadn't realised that it was as great a reduction as René stated. I should be interested to see comparative figures for Zeiss 40x NA 0.65/0.75 Phaco 2 objectives and for Phaco 1.

René's figure raises a question. Take a 40x Phaco 2 objective, then the maximum NA for phase contrast must presumably be well below 0.65. A traditional test diatom for such objectives is Pleurosigma Angulatum. This resolves with good contrast using phase contrast. However, it was originally intended as a test using brightfield and, maybe, oblique illumination. Hence, at least with brightfield, the condenser must usually have been stopped down to obtain adequate contrast. Thus, one is comparing like with like. It would seem to follow that techniques that allow full use of objective aperture are in fact producing far greater effective resolution than traditional standards and tests based on stopped down brightfield expected.

I should welcome comment. Selwyn

Although aperture is most often discussed, a Leitz Technical Bulletin outlines the importance of condenser "quality":
 Link : ../MicGroup/Misc/LeitzRes/

It's a different perspective, related to both resolution and contrast in conventional BF light microscopy.

-Keith

"A. S. St Leger" wrote:
> The issue of condenser aperture is interesting.

Hi Selwyn,
I find what you are saying very interesting.

If spatial resolution is d=1.22*lambda/(NA of condenser + NA of objective)
Then I would add an APO 40 with a 0.95NA to your list of seeing the resolution difference.

Looking at phase contrast and only subjective observation with what I have, I found better resolution using my Heine condenser-PV Apo L 40 than the Zernike condenser-Phaco 40. Does this compare with your results?

I think you are correct to just make comparisons with brightfield.
.....
Kind Regards, Charla Mason, Victoria Canada

Hi Charla,
I may not have followed your point so correct me if I am barking up the wrong tree.

The Zeiss 40x 0.95 NA objective is Phaco 3 and hence uses the condenser stop mentioned by René for 100x objectives (i.e. circa NA 0.65). Thus I am assuming that the 40x 0.65 using Phaco 2 has its effective aperture reduced by a similar proportion (does someone have details?).

The 40x 0.95 NA objective gives superb phase contrast; as expected a brighter clearer image (than 40x 0.65 NA) though some of the difference must be put down to this being a planapo lens rather than a planachro and not merely to increased NA light gathering. As to perceived (visual) resolution I am not so sure. I look mainly at live protists and don't find a lot of extra recognisable detail manifest as one moves from a nominal 0.65 NA to a nominal 0.95 NA and above. Looking at a prepared slide of Pleurosigma a difference between the two 40x objectives does manifest. This is in the contrast and there is suggestion of greater detail in the pore structure. Comparing those objectives at full aperture in DIC or DLC then the advantage goes more strongly toward the objective with higher NA though, as I mentioned, images of protists aren't particularly more revealing.

Incidentally, the imaging of living bacteria isn't much different between the objectives in phase contrast or DIC modes but this probably reflects their lack of internal structure at light microscopy resolutions.

Selwyn

Hi Rolf, Selwyn, Rene, Charla,
The big mistake that the Germans made in the 19th century was to downplay the importance of condenser aperture on resolution. Even Abbe was guilty of this, until it was pointed out by a very practical practioner, Robert Koch, that for all the superior quality of Zeiss objectives, the lack of a good aplanatic, achromatic condenser was preventing him developing the best images of bacteria!

The English, American, and French microscopes had no such disadvantage. The most thorough investigation was carried out by E. M. Nelson, with the conclusion that for most practical purposes, limiting the aperture iris to 75% of the full aperture of the objective gave in most cases the best compromise in terms of image resolution and contrast. However, Maurice Francon in 'Progress in Microscopy' points out that a small measure of over illumination, up to 118%, develops the maximum resolution in a theoretically perfect objective. This is the case in many objectives, but it does depend on the overall goodness of the corrections. Quite often, a 40x that with 75% gives a poor picture of the structure of Pluerosigma Angulatum, will with a small increase in aperture, suddenly reveal the structure. The margin is often very narrow, with glare increasing very rapidly past that point.

The situation with phase contrast is much more complex, as is also the case with annular illumination or, COL. Here the annulus acts as a Fourier spatial filter, the zero order of the illumination is suppressed, and there is an improvement in the apparent resolution of small, circular periodic structures. Our Victorian ancestors were well aware of the effect, if not, until later, the explanation. My late friend and colleague, Ted Brain, carried out a thorough study of the resolution of Zernike PC, and concluded that its capability to resolve fine detail was better than brightfield, because of the ability to resolve small differences in refractive index, which tended to be washed out with brightfield illumination. Pictures taken with Ted's Halo 1 microscope of semiconductor structures tend to bear that conclusion out.

A good condenser, aplanatic and achromatic is a pearl beyond price, and E M Nelson's scathing conclusions a century ago concerning the deficiencies of the continental microscope, are as true today as when he first wrote them.

Cheers, Merv

Ah yes Merv, I recently learned Olympus has apochromatic condensers in their repertoire nowadays. Apochromatic! I remember I saw them already on English stands in the end of 1800. With correction collar for slide thickness! Yes, the Germans screwed up there, Abbe would have turned in his grave if he knew how much his Abbe condenser is misused as a cheapo universal condenser... And the English let it slip through their fingers, what a waste.

René

Hi Rene,
Do I hear the ghost of Edward Milles Nelson chortling from a cloud as he manipulates his (ghostly) Powell and Lealand No. 1? Probably, he is quite entitled to do so!

It is a measure of Abbe's indifference to the condenser that he allowed his 1873 design to dominate for so long. However, Abbe always listened to good advice, and having been severely critised by Koch, he did produce an excellent achromatic condenser in 1883. They were, however, very expensive, and the equivalent from an English or French maker was much less.

The Powell and Lealand articles were of course, in a different league. It is interesting to note that an achromatic condenser for a Baker microscope in 1856 cost £2 -15s - this was for a No. 1 stand that cost £13 - 10s. A house maid got £5 a year and her keep!

Very Best regards, Merv

Dear Merv:
Very enjoyable and informative remarks on the history of the Ach. Condenser. In today’s world many of the contributions of fellows like E.M. Nelson have been forgotten. However, if you look at his splendid photomicrograph of Amphipleura pellucida found in Carpenter’s, The Microscope and its Revelations (frontis piece), you will see how accomplished he really was. Much work on this test-object has been done by this group in the last few years and to see Nelson’s image from the ca.1880’s provides a rather interesting comparison. I am always pleased to see members such as yourself remind us of the foundation built by men like Nelson.

Thanks and keep up the good work...
Sincerely, Jim

Dear Jim,
You are absolutely right, Nelson's photomicrographs are difficult to equal, even today, and are a salutary reminder of how little progress there has been in the last 110 years. But, as Uncle Albert used to remind us, all these things are relative! I have a huge respect for the pioneers of the past, and, as a development engineer myself, am well aware of what it cost to reach such level of performance.

What a pity that Tolles and Wenham could not have sat down together and talked, without the interference of two armed camps - I suspect that they would have shook hands in the most amiable of conclusions if that had happened - both were skilled to a level it is difficult to comprehend, and both were within the thickness of a piece of tissue paper in their world views. Wenham had built a career on pushing back frontiers - and Tolles' understanding was within easy reach of Wenham, if it had been clearly explained - he was already, very nearly there! I find it one of the saddest stories in the history of science. The book by Harold Malies was published in America by Microscope Publications Ltd, Chicago, Illinois, 1981, Library of Congress No. 80 - 83457, ISBN 0-904962-09-1. I hope you find a copy, it is a very intersting account by a master.

I am writing up the hot lacquer process, warts and all, and will post it and send you a copy to review when it is finished.

My very best regards, Merv

Dear Merv:
Thanks very much for the info on the H. Malies book; I will work this issue with determination.

You bring to mind the aperture wars, and you are very correct, I’m sure they could have come to very good terms if given the chance. However, powerful minds often clash when exposed to public scrutiny. I don’t think that Tolles actually wrote too many articles as there were so many fellows who took up the issue for him. Now Tolles is known as the father of homogenous immersion and Wenham is remembered for his innovative train engine designs and of course his beautiful binocular tube. I think that history has been rather good to both of them...

I look forward to the lacquer process….

Thanks.. Sincerely, Jim

[ Note: The MS-Word document 'The Aperture Wars - Research & Notes', as originally prepared by Jim in 2004,
  is available as a Group file. ]

Jean-Marc,

Some very good pics of A.pellucida - thank you very much for sharing them.

Tangling with this particular 'test specimen' continues to be a right of passage for many light microscopists!

Also, the comment: "... most important part of scope to see dots is the N.A condenser" is very interesting as there was a great discussion recently in regards to condenser aperture. This is now available in Topic summary format:
  http://www.kscitech.com/MicGroup/Msg_05/index.htm
This message thread finishes with Jim Solliday's reference to 'The Aperture Wars', and so some fascinating documentation on this topic along with that for an immersion objective discussion including Mervyn Hobden has been incorporated into the summary. I usually contact the authors to request permission to use such documentation, but in this case the authors are long standing group members and I trust that they find the presentation acceptable.

Regards, Keith

"hellangel333" wrote:
> I use my Leitz Dialux 22EB with 100W halogen and blue filter CB12.
> UKA condenser (universal condenser for DIC) with 1,44 darkfield
> condenser head.
> Leitz PL APO 63/1,4 PHACO4 use with personal dic prism
> Leitz NPL FLuotar 100/1,32 use with personal dic prism
>....
> The most important part of scope to see dots is the N.A condenser
> head, not the objective. I begin to see dots with a poor Leitz EF
> 100/1,25.

Keith wrote:
> Your comment: "personal DIC prism" is intriquing.
> Did you actually set up a DIC optical system yourself?

I suspect that is referring to individual prisms intended for attachment to a particular objective.

David

Hi all, and thank you,
When I say "personal Dic Prism", in fact I have some old Nikon Nomarski Prism from a Labophot 1 and Olympus scope prisms in stock. One of them works well with all my 40X-100X objectives. This prism is inserted in a slider that fits the lateral slot in my Dialux, for who they know the Dialux 20-22. However, I have also 25 and 40 ICT Fluotar objectives that fit the UKA condenser. In my opinion, I think the quality of my own DIC light is, the same as the original Leitz ICT.

Very interesting article indeed "aperture war"!

Best regards, J-M