In order to demonstrate the various methods of oblique illumination I've continued the videos. The third, using the same convex knife edge (after Piekos), is on science-info.net now:
  Link : .../pages/EDFWilliams/VIDEO/test_DLC_03.avi

[ Right-click on 'Link' to download the video. ]

This is an experiment to see what happens when the lighting setup is less than optimal (not according to Köhler) for normal high resolution.

The specimen is a strew of A pellucida -- KK material. The video starts after the knife edge has been adjusted for maximum contrast and with Köhler illumination set correctly. Then the filament image is moved upwards by focussing the light source up and past the critical position to some millimetres above the specimen. Then it was brought down again past Köhler to a position at least 50 mm before the specimen. The beam is already expanding, at this extreme setting, to such an extent that there is very little light getting to the specimen at all.

As far as I can see there is no improvement in resolution as the image of the filament is moved from Köhler to critical and a little further. When the filament image is moved the other way, contrast improves slightly but this may be due to a reduction of brightness. Then it rapidly becomes awful.

Don W

Nice demonstration Don. In the latter third of the video there was a suggestion of beading.

I wonder whether AP is too stringent a test until one has satisfied oneself about optimal use of the Piekos plate. Last night I was messing about with F. Rhomboides (KK test slide) viewed with a Zeiss dry 40x planapo (NA 0.95). F. Rhomboides is not an excessive challenge for this aperture. My aplanatic condenser was used both dry and watered to the slide (not much difference).

With DIC I was just able to make out the dot pattern across most of the specimen but the position of the analyser and condenser was critical and this was not easy to reproduce.

DLC, with the plate on the field diaphragm (as recommended by Barry Piekos) made resolving/contrasting the dots clearer and easier but again the setup was critical with respect to both condenser position and postion of the plate on the diaphragm. Tiny alterations to either lost the visualisation immediately. Also, position of the specimen in the field of view makes a difference. Further, the dimensions of the plate seem important. Mine was of mylar and had a convex edge.

Selwyn

The field diaphragm on this microscope is 370mm away from the condenser iris. To put a knife edge there is a difficult task and moving or rotating it would be impossible because it's totally enclosed in a solid aluminium housing. The best I could do (and did) was to put it across the window in the base which is about 80mm from the condenser iris. It gave poor results and that's why I put them just above the filter holder, a few millimetres below the condenser iris. I've tried to find increased resolution in the "shadow" and failed. I did find better contrast, but that's something we all know about.

I think the beading you see is also visible in the strictly Köhler image but the contrast is not as high. But I have mentioned, and posted pictures and videos, with rows of distinct dots imaged with oblique light using a knife edge and also an intermediate lens. None of this resolution can be attributed to DLC enhanced contrast. I still maintain it's just a convenient and efficient way of getting oblique lighting. And the specimen you use to demonstrate enhanced resolution must be capable of showing such -- and so you need A pellucida. But the diffraction effects of the lines has a serious effect on what you see. The light that emerges from those little gratings is polarised and all you need do to prove this is slide an analyser into the beam or pop one into an eyepiece.

Don W

Selwyn,
To follow up on Charlie's valid comment re things becoming confusing when we are discussing illumination techniques, in particular all the variants of oblique illumination, I think we should refer to DLC only when the edge plate is positioned per your experiment. Then DonW's experiments could be referred to as 'oblique illumination using a sub-condenser convex stop' per Piekos.

I'll be more careful with these distinctions in any summaries for the general "DLC technique". Obviously, comments from any group member that may assist with minimizing confusion would be welcome.

Do you have an approximate radius for the mylar stop that you feel gives the "best" results? Barry concurs that this radius is important.

Thanks, Keith

"selwynstleger" wrote:
> DLC, with the plate on the field diaphragm (as recommended by Barry
> Piekos) made resolving/contrasting the dots clearer and easier but
> again the setup was critical....
> Further, the dimensions of the plate seem important. Mine was of
> mylar and had a convex edge.

Keith,
I have used the convex edge of the mylar which had a radius of approx 4.2 cm. Previously I used a cardboard cut-out (covered in tin foil for sharp edge) of similar convex radius. My mylar stop is by no means satisfactory. I shall experiment with other variants w.r.t. x, y dimensions of the stop and straight v convex edges. I shan't alter the radious for present (mislaid my compasses). Everthing I am trying is with the stop in the position Barry recommended.

I reiterate that if DLC has any resolution enhancing properties then these are likely (based on my simple attempts) to be critically dependent on its dimensions, position on the diaphragm and position of the condenser (with the latter two parameters possibly interacting rather than being additive.) Thus, simply moving the condenser up and down may be insufficient. Additional variables are the position of the specimen in the field of view and, likely if a row of dots is being visualised, orientation of the specimen. Bear in mind that Barry's most intriguing examples were with non-linear structures.

Selwyn

I have been looking again at what was accomplished in the 1880s and there is no doubt that microscopists in both the Europe and the USA fully understood the use of oblique illumination and also its limitations with respect to introducing, in some instances, 'optical artifacts'.

I will not have hobby time to complete a webpage next week, so in the meantime will post an interesting photo of a Bulloch microscope (1883) with a versatile oblique illumination mechanism:
  Link : www.kscitech.com/MicGroup/I/Pic_10.htm

Also, for members interested in antique microscopes in general 'The Microscope Group Archives' have been updated to include a link to the Golub Collection at the University of California, Berkeley:
  Link : www.kscitech.com/MicGroup/General.htm

Regards, Keith

For the hell of it I tried a concave knife edge and the picture is the last one in the Don Williams photo album.

I can see absolutely no difference in resolution, contrast, or anything else. In fact it's better than many of the pictures made with convex and straight edges.

However, in this case the knife edge was about 100mm below the top element of the condenser and not on the filter carrier. All the other conditions were the same as the last of these pictures I posted.

I tried to put a knife edge (convex) in front of the field diaphragm, but it's so hot down there that the Mylar buckled and became useless. It needs to be metal for that experiment. And I'd also need a mechanism for translation and rotation rather tricky because it means major engineering such as cutting a slot in the lamp house. And it would be limited to about 100 degrees of rotation or a little more maybe.

Don W